I agree, no hermits. They'll knock stuff over and kill the snails.
I would do one Nassarius, one Cerith, one Trochus. Trochus grow slower than Astrea's, atleast that has been my experience. Possibly add one Nerite to that list, I can't remember how big this thing is anymore, I've read too many threads today.
Maybe a peppermint shrimp, a fire shrimp is probably not going to be happy in that tiny of a space, or a small PomPom Crab.