I'm not that sure, but I think you can attach a chaplain with termie armor or without a jump pack to an assault squad. Its just an (IMO) pretty bad idea because since squads move at the same speed as their slowest member. By adding someone without a jump pack, you're disabling the squad's jump pack ability, and in the case of termie armor, preventing them from making sweeping advances.
I like the metal chaplain and librarian models, but I would prefer them to be in plastic. It makes it much easier to swap out weapons and such.