this is the first time I've played BG2, despite being around when it was new... My question relates to how to choose other mages: I had nalia (but she left when it became evident that I was more interested in the sewer quests than her story). Are all of the npc mages that I will encounter void of the concern of subkits? Will they all be able to learn every spell that might be useful?

If by "void of the concern of subkits" you mean what I think you mean, the only mage that has any restriction in what spell school may learn is Edwin who is a Conjurer and can't cast Divination spells. This basically means True Sight, since all the other spells from that school are rendered obsolete relatively soon. In a full party at least.

The other choices you have are Imoen, Nalia, Aerie, Jan or Haer'Dalis and all of them are either dual class (the first two), multiclass (the next two) or bard (Haer, up to lvl6 spells though), all of which can't specialize in any school.

Jan can't cast Necromancy spells due to being an Illusionist. Edwin can't cast Divination spells due to being a Conjurer. Haer'dalis can't cast 7th level or higher spells due to being a Bard.

Apart from that, the only thing that would keep you from learning all the spells you need would be the mage's intelligence. Mages can only have a certain number of spells in their spellbooks, determined by their INT score. You can bypass this by drinking Potions of Genius or Potions of Mind Focusing to boost their INT up to 19 before they scribe anything new.

Choose them by what role they should fill in your party, not by what they can or cannot cast. Unless you're picking just one mage to take for the whole game, the lack of a few spells does not make a great difference, since your other caster can cover them.

This is the basic formula -

Imoen -or- Nalia -or- Edwin +and+ Aerie -or- Jan -or- Haer'Dalis

Oups, yea.. Slip of the mind... Was never too warm about that fella, so I forgot about his Gnominess.

Than you for the clarification, guys. That was my last obstacle in determining where to go in terms of party creation.