[VonDondu]

Having a Thief or a "tank" in your party would make things a lot easier. But that's exactly why you shouldn't bring one along: you might end up relying too much on your Thief/tank, which would defeat the whole purpose of playing a theme party. But it's entirely up to you.

If you need muscle or cannon fodder, summoned monsters will help. I'm not sure about Tutu, but in the original version of BG1, there was no limit to the number of monsters you could summon. It seems to me that being able to cast lots of summoning spells would make playing an all-Mage party very fun.

If I were planning to play an all-Mage theme party, I would use any (or all) of the existing NPCs who fall into the Mage category. They include Xzar (Necromancer, Evil), Xan (Enchanter, Neutral), Dynaheir (Invoker, Good), Edwin (Conjurer, Evil), and Quayle (Cleric/Illusionist, Neutral). That's certainly an interesting variety. You could also dual Imoen to a Mage, which is a very popular choice. Although her alignment is Good, she'll always get along with your main character no matter which alignment you choose.

If you want Dynaheir in your party, you'll have to keep Minsc in your party or find a way to get rid of him (which isn't too difficult). Same with Xzar--you'll either have to keep Montaron in your party or find a way to get rid of him. If you want to have a tank in your party, you might decide to keep Minsc or Montaron; and if you want a Thief in your party, you might decide to keep Montaron.

Your character's alignment should be determined by your party composition and your own playing style. When it comes to the alignment of the NPC Mages, there is a decided tilt in favor of Neutral and Evil NPCs, so Dynaheir seems to be the odd one out. She doesn't get along with the Evil NPCs--she complains about the way that Xzar "smells", and Edwin wants to kill her. Personally, I don't play with Evil parties because I don't enjoy playing evil characters, but if you don't mind playing an Evil party, then it might be right for you, and you might enjoy it.

As for party size, I think that six-member parties work best in BG1 just because of balance issues. In other words, BG1 was designed to offer balanced game play if you have a party of six who advance at the expected rate. If you have a couple of 10th level characters in, say, Chapter Five, the game is unbalanced because your characters are too powerful. I think this would especially apply to Tutu, which lets you cast a full arsenal of 4th and 5th level (and higher) BG2 spells including Stoneskin, Lower Resistance, and Spell Immunity which were not available in the original version of BG1. I'm not saying that it isn't fun to play extremely powerful characters; but BG1 was a lot more fun to me when it was so challenging that it was scary. Therefore, I would play a six-member party so that my characters would advance at the appropriate rate.

Besides, if you're playing a theme party, the theme is a lot more meaningful if you have a full six-member party instead of a two-member or three-member party.

I don't see anything wrong with compromising a little bit as long as it doesn't have an adverse effect on your theme. For example, I don't think it would hurt to have a Mage/Thief in your party to take care of all those pesky traps as long as you don't backstab all of the monsters. In practice, this means that you could have a character like Imoen in your party (dual-classed to a Mage), but you should put all of her thieving points into Find Traps and Open Locks (don't put any extra points into Stealth).

I've never tried a party like that, but it does sound like a lot of fun.