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, http://www.momminiaturas.com/divine-rage/ 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)