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Personally I would try to stay away from crabs. That coming from someone that has 4 tanks 2 with crabs and 2 without. IMO get only a couple of snails of each kind to start with and see if they live most die because people get to many and they starve to death.
As mentioned, nassarius snails are great, especially for cleaning the sand. Astraea snails are good too. Try some hermits; just make sure you have extra shells for when they get bigger, or they will kill each other for their shells.
Hermits also kill snails for shells. The astraea is actually one of their favorites because they can turn them over and the snail can't turn back upright.