So, in a few weeks I'm starting Might and Magic VI: Mandate of Heaven. I've completed Might and Magic I-V but the engines of those games are completely different. I know there's something in the game about choosing either a Light path or a Dark path. Can you acquire both Light and Dark magic or once you make your choice you're sealed off from the other? Which path is more powerful? I very much prefer to play a good guy but I also like to optimize my party to face every challenge with ease. Are there things I should absolutely know before starting? Glitches, etc? What's the most powerful party you can create? Which skills are essential?

Sorry for asking so many questions but I want my time with MMVI to be as smooth as possible.

Thanks in advance!

Yes, you can choose both. However in order to get the Light Master upgrade you need to have maximum positive reputation, and in order to get the Dark Master upgrade, you need maximum negative reputation.

The problem with reputation is that over time it goes back to average. So it's difficult to maintain maximum good/evil reputation.

Getting negative reputation is easy: you can kill innocents, or if you don't like to go that path, you can (without giving spoilers) in one of the dungeons do a certain evil thing that gives you a bonus and maximum negative reputation too. You can do the evil thing as often as you want.

Getting maximum Good reputation is more difficult. There are only precious few quests which let you have it, and once you finish them, that's it.

I suggest the following: play normally, then once you reach the Light/Dark Master teachers remember their locations. Then once you do the Good quest and get maximum positive reputation, quickly go to the teacher, get upgraded to Master Light and Bob's your uncle. Once you get maximum negative go to the Dark Master teacher to get upgraded to Dark (or kill innocents if you're impatient). Alternatively if you do the quest that grants you maximum good reputation before you find the Light teacher, then reload, and postpone finishing the quest till you know the teacher's location.

I can give you spoilers as to the location of Master Dark/Light teachers and the quest if you want.

Note that once you get upgraded to Light/Dark, you can use all the Dark/Light abilities without worrying about reputation. Arguably, the most "powerful" party is 3 mages and a cleric, because towards the end of the game magic becomes more powerful than might. However I strongly advise against going this route because:
a) It requires knowledge of game strategies beforehand, especially with regards to spells.
b) It might make the beginning of the game very tough, since mages are weak at the beginning (and the game is tough in the first areas as it is).
c) It might make the end of the game too boring. Hey, you have 3 mages. After a while every battle will have you just doing the same actions with all three of them.

Personally I suggest a balanced party. It won't be the most "powerful" but it will definitely be much more fun and let you experiment with the whole array of strategies, rather than just relying on magic. I suggest: Mage, Cleric, Druid, Knight. Yes, the more experienced players will say that druid and knight are "weaker" classes, but I believe it's more fun this way, and it's definitely a viable party to finish the game with.
The default one of Mage, Cleric, Paladin, Archer is balanced too, though in my opinion Druid+Knight is a more powerful combination than Paladin+Archer. In general though, Mage and Cleric are a must, and the remaining 2 characters depend on your tastes. You need all the mage and clerical skills for spells. If you have a druid, you can specialize: the druid can take one or two skills from the mage or cleric. Only mage and cleric can have light/dark, and again you can have one specialize in each.

I suggest getting Learning quickly since you don't get XP retroactively. Meditation and Bodybuilding can be left towards the end, if ever, because you do get HP/SP retroactively after learning it. Have your knight study Identify/Repair/Perception/Disarm.
As for weapons, I heard that overall daggers are best in dealing damage, because their speed compensates for the little damage per hit they do, so in the long run they are best. Personally however, I think it would be boring to have all your characters specialize in same type (and annoying if you find a powerful weapon that no one can use, because they're all doing daggers). Ultimately it won't matter very much. I went with sword for knight, dagger for mage, mace for cleric and staff for druid.
Get bows for everyone so they can use it, but you don't need to specialize in it. They are extremely useful in the beginning, but towards the mid/later stage of the game you'd rather have your spellcasters cast spells and your knight attack mellee for greater damage.

Diplomacy is the only completely useless skill. Completely. But... I did specialize with it because hey, I wanted to roleplay my cleric as a good diplomat.

To sum up: this game can be easily completed with a "sub-optimal" party. So don't worry about wasting a skill point here or a stat-increase there. You'll do fine. Enjoy roleplaying. Experiment with different spells and skills. Getting bows for everyone will make the first areas much easier. You can't buy them in New Soprigal, so consider going to Castle Ironfist just to buy them, then going back to do the New Soprigal quests.

The druid is almost overpowered in MM6, really. I think they toned the class down a bit in MM7 because of it?

You really can't go wrong with any party in MM6, though. I guess balance is probably what you would want to go for.

Knight, Druid, Sorcerer and Archer lets you cover all the bases. That is my current party too. Works like a charm.

I'd go with KDCM (knight, druid, cleric mage).
Knights become hit points by the end of the game. But for most of it, the Knight will get you home when others are goners.
Druid is powerful Open to both Elemental and Cleric Spells, the class gives you some neat options.
Cleric. With a druid you have two healers. Not bad for first timers (or old timers for that matter). And can do great damage in its own right.
Mage. All the neat spells in one package.

I don't like Archers because bows loose their usefulness pretty quickly in this game. Good early on, by mid-game, bows are used so as not to use up blue goo.

Knights are playable but not very powerful, in MM7 they got upgraded, in MM6 extra HP are the only thing they get. In MM6 all classes have the same physical attack power.
I'd say archer/druid/cleric/sorcerer is the most powerful party without duplicates on the long run, get bows for everyone ASAP.

Some enemies are completely immune to magic, though. So a knight certainly is useful now and again later in the game too. The knight is good for your misc skill character as well. He\she only needs Plate skill and a weapon skill and bodybuilding. Since the other classes needs plenty spell skills and the like, you'd want a character for Disarm, Identify and Repair.

The most powergame-y party is, as previously mentioned, 3 Sorcerers and a Cleric, though I think that would be pretty rough for a first playthrough. The default party of Paladin/Archer/Cleric/Sorcerer will put you through the whole game without a ton of problems, though you might also be served well with Knight/Druid/Cleric/Sorcerer. Just be warned that Knights aren't super great for anything other than taking hits and fighting the odd magic-immune monster by the end of the game, even with Greyface's patch (which I recommend) fixing the recovery speed of heavy armor.

The Light/Dark division occurs in MM7, not MM6. In MM7, you have to choose either Light or Dark.

MM6-specific details:
Any classes with the ability to pick up Light/Dark can have both concurrently. Be advised that there is only one copy of the "final" spell for each of Light/Dark, so only one character in your party will ever have Divine Intervention in his spell book, and only one character in your party will have Dark Containment in his spell book.

Light and Dark magic have different spell lists, and both have very useful spells (ex. Day of the Gods is Light, Day of Protection is Dark). Dark is more powerful for straight up damage (shrapmetal, dragon breath), while Light is better about the buffs and protection; overall you'll want both available at a good level.

Perhaps the most important glitch to be aware of: If a character has more than 400 in a stat, and they are hit while in real time mode (doesn't apply in turn based), then the game will crash. This will probably never be an issue unless you do some massive stat focus on someone, including high stat bonus equipment and high level stat buffing spells (namely Day of the Gods with a large skill level in Light).

Perhaps one of the most important game issues to be aware of: Getting Light Magic Mastery requires "Saintly" reputation. It is possible to lock yourself out of being able to get Light Master, if you act evil and you've already completed all the quests which provide reputation bonuses. You can't donate your way to that level of reputation (though you can get close). Don't go for Master Dark ("Notorious" reputation is easy, just Armageddon towns until you've got the right rep, get Dark Master, then pay the fines and donate at a temple to improve your rep) until after you've gotten Light Master for everyone. There will be ample opportunity to buff your reputation to Saintly, and you can't get Light Master until later in the game, but you should probably make it an objective as soon as you are able to survive in the area where the teacher lives.

Skill commentary:
-For me, Expert Air magic is pretty much always my top priority when starting a new game, for the extra data that Wizard Eye will reveal (items on the ground).
-There are a number of useful spells scattered among the various magic skills. Mind is probably the weakest of the magics, and you can wait to advance it until later.
-Light/Dark are very strong, but also very spell point intensive.
-Disarm Trap will be useful all game. It doesn't need much more than Master, but someone should advance it.
-Merchant will be useful until sometime after mid game, when you start rolling in money.
-Diplomacy is useless. Learn the skill if you want, but don't ever add skill points to it.
-A number of the skills should only be advanced by one character. For example, only one person can open a given chest, so only one person should put points into Disarm Trap. Only one character can interact with a merchant for a given purchase/sale, so I'd say only one person should advance Merchant; everyone should learn Merchant, since it helps discount Training costs, but I'd only have one person Master it.
-Learn the Learning skill as soon as you can afford it. It is expensive, but hey, bonus experience points after that.

Party Commentary:
-The "most powerful" party depends somewhat on what you want to do well. As I understand it, a party of 3 sorcerers and a cleric is considered the fastest way to kill monsters, but I don't advise it for a first time play through. Sorcerers are squishy.

-If you take a Knight, you'll have a big HP pool that Shared Life can donate to other party members, someone who can do physical damage and while wearing strong armor, and there will be no question about who will take care of all the misc. skills for your party (the Knight won't be spending any skill points on magic, so let him take care of all the misc skills while your casters work on magic skills).

-A cleric or druid will be ideal for primary heals. A paladin doesn't have enough skill points to be a primary healer.

-A sorcerer or druid will be ideal for outputting magic damage, and for the utility Elemental spells.

-A druid can cover any Self (cleric) or Elemental (sorcerer) magic areas, but they don't have Light or Dark.

-A Paladin is a hybrid Knight+Cleric. An Archer is a hybrid Knight+Sorcerer. A druid is a hybrid Cleric+Sorcerer.

-I suggest taking two characters that can cover healing, so someone is around to heal the main healer if the main goes down. A paladin is sufficient for a backup healer.

-Take a mix of classes, so you don't have several people competing for the same kinds of equipment. If you have 4 sorcerers, everyone is going to want leather (meaning all that chain and plate goes to waste) and daggers or staves (all those swords, axes, spears, maces, etc. go to waste).

-I suggest taking a paladin (plate, wide weapon selection, back up self magic), archer (chain, decent weapon selection, elemental magic), cleric (primary self, chain, mace), and sorcerer (primary elemental, leather, dagger OR staff). Split Light and Dark advancement between the Cleric and the Sorcerer, with one class focusing on one and the other focusing on the other (your choice which focuses where). Paladin and Archer should split the misc skills between them.

Alternatively, a Knight, Archer/Druid, Cleric, Sorcerer will also work well. If you swap for the Knight, consider taking the Druid as a back up healer. The Knight will take care of all misc skills for this party, leaving the casters free to focus on magic.

Ah yes, I forgot about this. All the more reason to specialize: have cleric and mage to specialize on dark/light each.
Also, edited my post to make it clear that you only need maximum positive/negative reputation to get Master light/dark magic. Thanks for reminding me of this.

Also, since no one mentioned this yet: companions. Make sure you get good ones. They reset when you reload your game, so if you don't think it too cheesy, you can keep reloading till you get the ones you want. I went with Instructor (+15% experience) and Teacher (+10% experience), but consider getting a scholar (+5% experience and unlimited item identification) which will save you a lot of skill points since you won't have to spend any on Identify skill. For a full list, check here:
http://tartarus.rpgclassics.com/mm6/hirelings.shtml

The druid gets spellpoints from both Personality and Intellect. Thought it was worth mentioning. With the meditation skill they will be swimming in spellpoints. Even more so than the Sorcerer.

Quick clarification on the above: the peasants around town are randomly generated on load (the peasant "spawn points" are constant, the portrait, name, and profession of the peasant will be randomly assigned when the area is loaded). Anyone you've hired doesn't change.

A scholar is good early, but I'd get someone with a higher bonus later. I also favor the Teacher + Instructor, just as soon as I'm out of the initial cash crunch.

If you have an open hireling slot, then a factor or banker is an easy filler. After the initial hire cost, they literally pay for themselves and will quickly make up for their hire cost.

*this contains spoiler*

i like 3 mages + cleric, they can dish out the best damage and they provide all the utility spells. My air mage once one-shoted even a dragon

To get great results you need to specialize each mage in 1 school. Various schools are better cause some monsters are completelly imune to some spells. (there are some rings and amulets increasing magic schools that make your spell about 50% stronger)
Fire mage, Air mage, Water (for utility spell) + Dark mage, Cleric with body + light magic

Though mages are glass cannons, when you get loyds beacon you can jump to the temple in the middle of the fight heal yourself (get there mana too) and jump back into fight.
Flying is very useful, enchanting too. Dark magic provides spell increasing magic resists, white magic increses basic stats and gives you another melee boosts. Even mages can do decent melee damage with them.
You can use combos like fly + meteor storm to kill all on the ground, or enchant + finger of gold to have enough gold (leveling is pretty costly later in the game)

I won it also with 3 dark mages and 1 dark cleric. it is difficul to manage at the start cause you get dark magic later in game and the spells need a lot of mana. But later you can just run and mow everything with sharpmetals.

The mages get some cheap spells at the beginning like fire arrow..etc, they are enough to kill weak monster untill you get better spells. You just need to fight in turn base mode (press enter) then the monsters cannot come so fast to you and damage you.

Red dragon? Implosion or starburst?

My fav group in MMVI is always SSSS. Not when going for speed runs, but just for general play. I just feel like there's no point bringing in a Cleric at all.

I generally assign one to Light (for early-game Hour of Power/Day of the Gods), one to Dark (for early-game Day of Protection), one to Air, for early Fly, and one to Water (where I funnel all horseshoes etc for Beacon/TP).

Due to early-early game being hilariously weak without the Shrine of the Gods TP exploit, which I refuse to do, I start with exploration quests, then transition into looting a few late-zone chests for the chance at end-game daggers/leather just to make early-game easier, once I'm strong enough for the explosions to not one-shot the group.

As soon as you get Hour of Power and Beacon, the game's basically over. Even with rubbish daggers and almost no spellpoints or offensive spells on 4 Sorcs, you can breeze through the early-mid dungeons. You gain levels insanely fast too.

After you're at Saintly for Light Master (which isn't really necessary, but I hate having to cast Hour of Power every few seconds to avoid Weakness), you dive into Dark on each character. I tend to keep Dark and Meditation around the same level for each character, and stop around 20 on both.

Shrapmetal is obviously the way to go. The only times Shrapmetal's weak are against the Eye packs in Darkmoor really. I'll switch to Dragon's Breath for those, as they can easily 1-shot the Flying/Terrible Eyes, and in The Hive, where you just don't have enough mana to spam it. But hey, that's what Blasters + Hour of Power's for, right?

I don't invest whatsoever into Elemental Magic other than what you need for Flying, and 12 Water for Beacon. Those 2 mages can easily take care of the Oozes in Werewolf's Lair and Iron Mines.