Using FW bull centaurs as grotesques is often done, and if not for the price I would probably do the same that you suggest for the black souls/immortal guard. I think that as long as you have a coherent paint job the army should look fine, it's not as if you were mixing models in the same units.

Also in case you didn't know of them already, as some evil dwarves models that might also be used as black souls/immortal guard (depending on how they look next to Mantic own immortal guard)