I want to play a ranger/cleric or a fighter/mage Is it better to multi class or to dual class this when you play from beginning to end in BGT?
If dual, I will play a fighter or ranger until level 9 in BG2.
Dual classing requires a lot of preparation beforehand to get the most out of it. But I believe the greatest drawback to it, is losing access to the class that you dualed from until exceeding the level with the new class. Depending on how high a level you dual your class at and how many characters you have in your party, you could spend a lot of time feeling pretty useless.
At level 9 though and you should be about level 8 after BG1, you should find that dual classing is fairly painless as you will only struggle for a small part of BG2 (especially if you keep your party small). Once you hit level 10 with your second class and regain your first class abilities things will become a lot easier and as BG is experience capped rather then level capped. Because of this you will not lose many levels of the new class you dualed into.
Fighter 9 / Mage 30 (the maximum level for a mage is Level 31). Level 31 does not grant any more spells to your spellbook so a Fighter 9 / Mage 30 can cast exactly the same spells as a Level 31 mage except he has way more HP and is pretty handy with a melee weapon...
Ranger 9 / Cleric 39 (out of a maximum of 40 levels) all spells granted, more HP then a straight Level 40 cleric However
The cleric's chance to hit is higher then the ranger class, so by default the highest Thac0 is used which is from the clerics class. you do get the bonuses that come with being a ranger, and by dual-classing at level 9 you will get an extra 1/2 an attack a round vs a straight cleric and you can become specialized in a weapon unlike the cleric giving you an extra 1/2 an attack so that you can attack twice a round. Dual-wield and you get a third attack.
Dual-classing also allows you to take advantage of the specialization fighter classes like the Beserker and Kensai.
While I'm not familiar with the Beserker, the Kensai gets bonuses to his/her armour class (-2AC) as well as an increased chance to hit and damage (+1 to hit, +1 to damage every 3 levels) and their initiative increases with a weapon as well so that they might swing that big weapon a bit quicker then the enemy might in a round (-1 to weapon speed every 4 levels). Finally they get an ability called 'Kai' which maximizes the damage dealt by a weapon for 10 seconds (every 4 levels this increases the amount of times you can use this per day).
As a fighter/mage you cannot use armour without disabling your spell casting (with a few rare exceptions), so the Kensai's bonus AC is a welcome addition. Finally, your Hit Dice HP/level drastically reduces after level 10 (+3HP/level) so having a Level 24 Fighter/Level 20 mage wont grant you much more HP as at first would appear then say a Level 13 Fighter/Level 28 mage. Fighter chances to hit also cap out at Level 21 (Thac0 is 0 at the lowest), so Fighter Levels 22-24 are a bit of a waste on a multiclass character.
Kensai -> Mage is most powerful but a Kensai in BG1 is too limited so it will be a standard fighter.
Is this the GOG version you are playing or the remake? Mod's would enable you to play the Kensai class in BG1, normally you cannot choose to be a Kensai in BG1, only a fighter. Having imported my Fighter from BG1 into BG2 you can then choose to convert him/her to a specialist fighting class (so my standard armour wearing fighter became a Kensai).
If I take fighter/mage, a gnome would be the best choice, I guesss. Thoug losing finger of death and horrid whilting is not so great, getting more int and spell slots is.
For ranger/cleric, there is only one possible race for each dual or multi.
As an illusionist you do lose a few good spells over a standard caster, however you have available an extra slot with some very powerful debuff/buff spells that might of been overlooked. Power word blind (no save unlike horrid wilting - so will hit all enemies every time (except magic resistance of course)), Maze also no save. Don't forget spell trigger, 3 instant cast spells up to level 6 that you can memorize. And if you want to break the game, I'll take my Project Image over finger of death any day (make a projection of yourself that can cast all your spells), create 3 projections have 3 free mages with your entire spellbook at your command there is some good info on this website
*EDIT: you can only cast one projection at once as you are basically held, sorry for the misinformation*
I want to keep Imoen a pure thief in BG1.
Will I get problems when I get her back if I change her to mage at the beginning of BG2?
Imoen will be dual-classed to mage at the beginning of the game regardless (Level 7 Thief, Level 8 Mage). She isn't imported into BG2 with the dual class you may of chosen for her (I went pure thief for her in BG1 which bought her to Level 8 after TotSC, but low and behold she lost a level and decided on becoming a mage).
I apologize for the insanely long post. I think i'm on a bit of a BG2 nostalgia buzz.