More like Neutral, then actualy good ...
And no, they cannot be corupted ... sadly.
Question here is what do you mean by "fitting for an evil playthrough" ?
Are you looking for another companion that will like your evil choices?
Or do you satisfy with the fact that s/he would not leave you?
IF you want first option ... all of them will do, just not all the time.
Shadowheart even likes some tricks and betrayals, she just seem to dislike unnecesary cruelity without purpose (or at least that is how it seemed to me) ...
Wyll dont seem to care about evil deeds, as long as they dont damage his "legacy" ...
Gale ... will probably be the worst choice for you. :-/
About the second option ...
I never managed to loose any companion, except Wyll ... and Gale, once in the long past.
Im pretty sure that they only leave you as reaction for some BIG decision you make ...
Wyll leaves you if you side with Goblins ...
Gale, if you refuse to feed him an artefact ...
I never managed to get rid of Shadowheart, even tho i tryed.
