Yeah, those numbers would probably be on the upper limit of what I'd aim for. You won't necessarily want to add them all at once, as you'll no doubt be slowing adding fish over a period of time. You may also want to add them in stages, ie: trochus and turbo for the sides of the tank to start, then astrea for the rocks and finally cerith for the substrate (as you'll want to make sure there's some nutrients there for them).
It takes a pretty hungry and determined hermit crab to kill a snail - especially since they will be hard-pressed prying most of them off the rock or sides of the tank. If a snail dies they will of course avail themselves of the empty shell, but it's not always necessarily a good fit, either.
I'd get a Halloween hermit crab and a few blue legged hermit crabs (2-3), or maybe a few blue legged and scarlet hermit crabs. The algae eating crabs are cheaper but I find they tend to be more destructive in terms of knocking things around in the tank.