You can survive without a tank in your party. Missile weapons, wands, spells, summoned monsters, and the occasional melee attack should get the job done, especially if you can cripple your enemies with spells such as Web, Hold Monster, Emotion, Chromatic Orb (it stuns opponents at higher levels), etc.
A Thief is more difficult to live without. If one of your characters starts with Strength 18, you can use the Manual of Gainful Exercise to raise it to 19, which will give you a fairly decent chance to force open locks. Unfortunately, the tome is not available until late in the game. Knock spells get the job done, so maybe you don't need to worry about locks as long as you rest frequently to refresh your spells. If you have a Cleric in your party, you can cast Find Traps. Summoned monsters can set off some of the traps in BG1 (although I'm not sure how well it works in Tutu). If you have to walk over them yourself, Mirror Image will protect you from most forms of damage. If you don't have a Thief in your party, having six Mages in your party is probably the next best thing.
Healing spells are useful, but they are probably overrated. As a rule of thumb, if you don't take any damage, you don't need any healing.
And you can usually recover most of your hit points if you rest long enough.
I don't think I'd want to play a Sorcerer in BG1. Have you seen the spell progression table? You won't have enough spells.
Imoen does not have any conflicts with any of the NPCs.
You might consider the following party:
PC: Conjurer (Neutral or Evil)
Imoen: Thief dualled to Mage (make her a Specialist Mage if you can do that in Tutu)
Edwin: Conjurer
Xan: Enchanter
Quayle: Cleric/Illusionist
For the sixth party member, you can choose from Xzar, Tiax, or Safana (she can dual to a Mage if you give her the tome that raises Intelligence). You might also consider adding a Bard to your party, but personally I don't really care for Garrick or Eldoth.
Have you considered any of the NPC mods that are available for Tutu? I don't know much about them, but they might offer some choices that are even better.