I say you need less hermits (by 2) and more ceriths. I'd have about 4 or 5 of ceriths, nassarius, and astrea's.
Margarita snails are not tropical snails. They come from cooler waters and will usually die after very long. They're in cooler water for about 3 months or so of the year, so they may live about that long. Sucks though because they're really good cleaners.
The hermits...i'd take them out because for one, they arent nearly as good at cleaning as all my snails are, and for another they kill the snails for their shells. Even if you put in extra shells, you're gonna have problems w/them killing the snails eventually. I've had about 4 or 5 different hermits and each one has ended up w/a cerith shell even though i have tons of extra shells.