[llamabeast]

Yeah, I do think that making them immobile might make them slightly boring (more like research factories and less like units), but on the other hand I do like the idea that they swore never to leave their books until the Aufklarung consumes the world. Well, they'll be good either way, but personally I'd be inclined to let them move. If they did have only random astral, then to the small extent that they were able to help out with communions it would be thematic (since sages etc are also capable of acting as communion slaves) and fun (nice to have another option every now and again), while definitely not overpowered.

I don't think you particularly need to worry about differentiating them from the black priests though. They'll be different just because of the fact you'll use them very differently. A librarian/white acolyte would be a rubbish wandering preacher because he'd be too expensive for his holy level, and also wouldn't generate penitents.