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