Personally, as long as it's not one magic is 100% better than another, I don't see why we shouldn't allow at least two slayers of various types in each guild as long as how they obtained said lost magic is fairly believable. Maybe limit it to one god/devil slayer per guild though, so we don't have four of them running around...

Meaning even Ceri could defeat a god-slayer if she gets stronger/same rank as a slayer.