Looks like what 14gallons? I didn't see volume anywhere in your post but if so I'd say 2 - 4 Mexican Turbo, Astraea or Trochus snails to work the rocks/glass. A dozen or so Cerith or Bumble Bee's to stir the sand and a few dwarf hermits. The dwarfs hermits shouldn't mess with your corals too much but they could. I find my dwarfs are fine but my regular ones do knock over things that aren't glued down.
Avoid Nerite snails as they tend to be jail breakers and climb out of the tank. You can always add to it but you should't need too much more if you keep up on Maintenance.