The only crabs I'd add would be scarlet, blue leg, or mexican hermits. Emerald crabs are usually OK. I'd say 6-8 hermits, 6-8 astrea, nerite, or turbo snails, and maybe 10 nassarius snails. You could also have a serpent star if you wish. Stay away from the sand sifting stars. If you want to go with snails only, just up the snail count by 3-4 of each type to make up for the crabs. A pair of peppermint shrimp and, later when you're sure the tank is stable, a cleaner shrimp are good additions. Sally Lightfoot crabs are usually OK in a reef, but they do get large.