I feel the same about some of the characters whose story is tied directly to their class, specifically Wyll and Shadowheart. However, I think any arcane class would probably be fine for Gale, and the other two really don't have any of their personality wrapped up in their class.
le'eazel = Her goals in life would conflict with her being a wizard, rogue, and a few other classes. The fact of every word out of her mouth is abrassive, egotistical, abusive, and threatening to some degree or another means she can really only be 1 of 2 things Fighter, or Barbarian. Then there are a few other things that would be utterly stupid to have on another class. her skills as well are made for being a front liner/damage dealer in melee
Ast = slimey, but charismatic, a flair for dramatics, every word, even the way he talks screams only Rogue, or Bard. his skills as well make him a rogue, not even bard.
so every companion is built around their class or one nearly identical. yet if you toss them into that nearly identical class they wouldn't be worth much.
So no seriously keep them just the way they are.