I think the initial class and race should be there for the dialog options around that character. Since multiclassing is added you can in a way respec your origin companion to what you want. However, if Larian has made it possible to maintain the origin character's background story with a different class then I might consider respeccing the classes on some of the origin characters.
E. g. with Astarion you can get him 2 levels of rogue and the continue as a gloom stalker ranger if you want him to excel in ranged combat.
Laezel could e. g. get levels of cleric if you don't want to use Shadowheart.
However, I would definitely like to be able to respec the stats and starting spells of my origin companions so they will better suit the feats and ability increases I'm selecting for them.
Shadownheart needs to start as a Shar cleric with the trickery domain because her story is built around that.
Wyll is a warlock who has made a pact with a fiend. Otherwise his story would fall apart.
Gale needs to be a spellcaster for his background story vs Mystra to work. Maybe he could start as a sorcerer instead of wizard.
Karlach has her rage that she should have as a special power if she was specced out of being a barbarian. She is a melee character so if you select something else then a fighter, monk or paladin should be her choice. I guess her background story as being a front line fighter in the blood wars can still work with her as e. g. a fighter.
Laezel and Astarion are the ones who could start in a different class without too much impact on the background story. Laezel acts like she is a melee character, but you could e. g. respec her into being an assassin rogue or something. Astarion can from his background story be almost any class. I don't see him as a paladin, though.
If we have full freedom I would probably end up playing something like this:
* Me (paladin) * Shadowheart (cleric) * Gale (sorcerer) * Karlach (rogue or gloom stalker ranger)
