it's best to get a divers clean up crew consisting of several different types of snails and few if any crabs.
for snails go with the following:
Trouchas 25, absolutely the best algea eaters and they can turn themselves over if they need to so they last a long time.
Nassaurius 30, great sand bed cleaners. They bury themselves and come up when they smell food.
Cerith 15, good all around cleaner and doesn't die when it falls on it's back.
stomatella 10, great cleaners and they reroduce like crazy so you grow your own future cleanup crew
Astrea 20, cheap and good cleaners but they die when they fall over, but they do also reproduce fast.
turbos 4, nice to look at and good cleaners but pricey and they are natures bulldozers and will knock over anything in your tank if they want what is under it.
for Crabs:
scarlet reef hermits 20, won't bother anything in your tank and won't kill your snails for their shells.
all other types of crabs are risky as they typicaly will go after your snails and sometimes corals.
Look to get aprox 1 cleaner for each gallon