Here's my current party.

ME, h/e druid/fighter. (offensive spells)
Minsc
Anomen (healer)
Aerie (romance)
Jan

and soon to be Keldorn. But my question is, who should I have as my offensive mage (mostly evocation/summons) and as my defensive mage (abjuration, transmutation) ? It's between Jan and Aerie. But if this doesn't work out than I am thinking about going with;
(i'm just outside of Irenicus' dungeon)

Me, human abjurer
Anomen (healer)
Yoshimo
Minsc
Keldorn
Jaheira (romance)

Then having Imoen later instead of Yoshimo and having her with a bunch of offensive spells.
What do you guys think?

Edwin is a good offensive mage
and Jan has a lot of defensive spells
keep Aerie for her Clerical healing and protective spells
but keep Jan and also pick up Edwin
Anomen can also be a good tank if he has the right equipment

I wouldn't play an abjurer, but that's just because there are some nice transmutation spells.

Why does each mage have to be specialized? An offensive mage with tons of protections up is able to stomp any enemy in the game solo.

I wouldn't take Edwin. You need more melee power. Your initial plan of taking Keldorn is a good idea.

keep Aerie for her Clerical healing and protective spellsShe's also an offensive force to be reckoned with if you give her the right spells.

You want a single class mage for offensive magic as most missile/area spells have effects based on character level.

To be honest the character that can cast the most damaging offensive spells is Haer'D, because his effective level will always be higher than a mage on the same XP. He can got into defensive spin and just sit there casting spells and not get hurt while your melee fighters finish off the rabble.

Both mages should cast some offensive/defensive spells otherwise if one is taken out of the fight then you lose that ability.

Clerics have some great party-wide defeensive spells you should utilise. Druids have probably the best anti-mage spells in the game. You seem to have both of these covered in either party - personally I prefer druids to clerics in BG2, especially against mages and there are a lot of those.

Imoen will be a great mage when you eventually get her, but will be lacking in levels. Edwin is the most powerful mage, but Nalia is the best GOOD mage in the game and is better than Imoen because of the XP she gains when Imoen is captured.

Aerie can be awesome and has some amazing spell combinations. You can mix her offensive divine and arcane magic to great effect. Try doom-doom-doom-chromaticorb against a dragon and watch the effect (awesome). She is very under-rated, but if you use here right and actually use her cleric/mage spells in combinations she is just devastating and can actually cast more spells than any other character in the game.

To be honest the character that can cast the most damaging offensive spells is Haer'D, because his effective level will always be higher than a mage on the same XP. He can got into defensive spin and just sit there casting spells and not get hurt while your melee fighters finish off the rabble The only problem with that is many spells cap off at level 10 for damage:level.

True.

There are a couple of good ones that have higher levels before being capped - chromatic orb, flame arrow, skull trap, sunfire.

Also, a lot of the area effect, confusion and summon spells have durations/ranges based on level and the bard excels at these as well.

You do however, need a mage for the higher level spells of L7-9.