I would say Gale is canonically an Evocation Wizard, but will later multiclass into a Fiendlock if you ignore him long enough, probably Pact of the Tome (bit of a bookworm), good Gale is a pure Wizard,
I would multiclass Shadowheart into Paladin after the triais, since personally Dark Justicars seem more like Shar's equivalent of a Paladin than a Cleric, Oath Breaker would fit gameplay wise as well as Vengeance, Good Shadowheart becomes a Life Cleric of Selune aka the Moon Witch.
Durge is canonically a Storm Sorcerer who can later multiclass into Assassin Rogue, to help him kill more discreetly, canonically I would like Cleric of Bhaal instead of Assassin for Bad Durge, Good Durge probably becomes a Paladin as a form of redemption.
Astarion is an Arcane Trickster, just have it based on Vampire Powers and Thay Necromancy,
Wyll is Pact of the Blade, then have him multiclass into Champion Fighter, I mean he'll lose his powers in 6 months, might as well have him train and prepare to be the Blade without his pact powers,
Minsc is a Rage Ranger (Like Drizzt Do'Urden, Rangers seem to have a bit of anger issues, there should be a Ranger subclass themed after them).
Boo is canonically an Assassin, going for the Eyes is instant death.
Mithara is a Vengeance Paladin but Oathbreaker fits her more since she betrayed Lolth like a Heretic and then she betrayed the Absolute, (Double Oath Breaking)
Jaheria is a Circe of the Land,
Halsin is a Moon Druid with 1 level of Bard, (How else do you explain his sudden horninest),
Karlach any Barbarian class fits her.
Canon Tav was a Human Cleric of an unknown deity, who later joins the Absolute under the name Zealot Malik after Larian insulted him into joining the Dark Side.
