Other than occasionally eating a snail, crabs are fine. I started with a dozen blue and a dozen red in my 90. I think I had about 175 snails of different types. Search the web for reef packages, but don't believe the "1 crab/gallon" philosophy
I have a 135-gallon reef and 2 large hermits, a dozen reef hermits (small with red legs), 4 Mexican turbo snails and 4 nassarius snails. Seems to work okay - I would start with a few snails/crabs and go from there, I don't think you need tons of them to keep your tank in good shape.