Your party:
PC Fighter - Warrior
Minsc & Boo - Warrior
Anomen - 1/4 Warrior | 3/4 Divine Caster
Yoshimo - Thief
Jaheira - 1/2 Warrior (Tank) | 1/2 Divine Caster
Jan - 1 Thief | 1/2 Arcane Caster (due to his items i count him as a "full" Thief)
3 3/4 Warriors (including a Tank), 1 1/4 Divine Caster, 2 Thieves and 1/2 Arcane Caster. You should do just fine, but i would lose a Thief. I'd say drop Yoshimo for Aerie, Nalia or Haer'Dalis so you get another Arcane Caster who can dispel protections, cause some damage and such. Then, when you go to Spellhold you switch Yoshimo back in and swap him with Imoen on the island.
My current party:
PC Undead Hunter (Paladin) - Warrior
Mazzy - Warrior
Valygar - Warrior
Cernd - 1 Warrior | 1 Divine Caster*
Aerie - 1/2 Arcane Caster | 1/2 Divine Caster
Imoen - 1 Thief | 3/4 Arcane Caster
*Note that i have the "fixed" Shapeshifter kit installed which allows you to cast while Shapeshifted and shapeshift simply by equipping werewolf paws (no daily limit). This makes Cernd borderline overpowered, giving you a powerful Warrior with immunities (25 or 50% magic, non-magical weapons), a very low AC (-13-ish) as well as full Druid casting capabilities (Insects, Iron Skins, Nymph Summoning, Elemental Summoning). Oh, and did i mention Great Werewolves regenerate 1 hp/3 seconds? :devil:
I have 4 Warriors, 1 1/2 Divine Caster, 1 1/4 Arcane Caster and 1 Thief. Everyone can cast spells. It is going very smoothly.