I did an all mage party in BG1 using 3 PCs (conjurer, T/M, & C/I) and 3 NPCs Xzar,Xan, and Edwin - mageling madness - I had originally planned to take it on to BG2 as well - it was fun at first but the micromanaging of six mages for every battle eventually got to me and I abandoned them...
You can see details of their journey here -
Mageling Madness
An all mage type group in BG2 would be much easier I think since you start at level 7+ rather than level 1...I would probably still want a T/M and C/M tho...