I myself have about 2 assasins per province and a few inquisitors. I keep at least 1 assasin in each province and train the others on my own punk emissaries and buff them up to about 3 star, then i start takin out other inquisitors...needless to say...i have over a dozen kings killing assasins. Many a pope and king they have killed. It's also helped me gain quite a few catholic provinces easily. Emissaries are kinda handy too for bribing armies that have tons of enemy troops that don't contain an heir's unit. To me religion plays a somewhat small part in my conquests compared to assasins, but i do use a few cardinals.