will crabs/shrimp do more harm than good.
Crabs are predators. Hermit crabs will go after your snails or each other for the shell (new home). I'd go with a selection of snails, a fighting conch, a brittle star and shrimp.
Figure on about 1 critter per gallon:
Here's a list I kept around from a previous post from Logan.
Nassarius Snails : Great detrivores and sand-stirrers. They will hide in the sand until feeding time.
Cerith Snails Will also breed in your tank. Good algae eaters and sand-stirrers.
Banded Trochus : Algae eating machines. Ideal size, won't knock over corals/rock, and can move very fast.
Nerite Snails: They may crawl above the water line of your tank as they often become exposed in the wild during low tide.
Margarita Snail - Margarites pupillus Another snail to add to the algae eating arsenal. Will cruise around on the rock and glass.
Fighting Conchs : Little vacuum cleaners. One per 2 sq. ft. of sand. They'll eat diatoms and, sometimes, cyano
Cleaner Shrimp - Lysmata amboinensis
Snails need acclimation time to adjust to changes in salinity and temperature. Dr Ron's advice is, "Remove them from the bag. Let them dry out in a cup or bowl for about 15 minutes. Then take them one by one, and place them in the tank at the water line with the aperture next to the wall. The animals will extend, fasten to the wall and move into the water of their own accord. In effect, they will self-acclimate."