well, if you really want Sarevok in your party and keep the others intact, you may consider dualing him to mage since you've already have enough tanking power (Minsc + Keldorn + Jaheira), and you only have Imone as mage. Personally, I like to have more than one mage in the party and so this suggestion.
Or keep Sarevok as figter and kick out Keldorn (since I reckon you want to take Minsc all the way from BG to the end), and add either Jan or Aerie (Nalia is also a possibility, but you already have a thief -> mage in Imone) into the mix as another mage.
If you can supress the temptation to get Sarevok in (you can always wait for another game), I'd suggest that you just add Aerie to your current mix; not only she'll grow into a great spellcaster, but also Aerie + Minsc + Keldorn is a nice small group
