I second the variety of snails, but disagree with skipping crabs, unless you just don't like crabs. Each of the snails will have there preference on what they eat. Many of the snails will reproduce in the tank if a food soure is available. In my experience though, stay away from turbos and astreas, they don't reproduce in the tank and the astreas can not right themselves when they fall. Astreas and turbos are also more likley to be singeled out as there shells are prefered. Ceriths and smaller snails are more likely to get by.
Yes, the crabs will occasionally eat the snails, sometimes for the shell, sometimes for the snail. Snails starve to death easily in our tanks as we get an algae cycle, stock up on snails and then the algae goes away, not because the snails are eating it, but because the tank and/or hobyists tend to mature with time. Starving snails make easy targets I guess. The crabs also need the shells as they grow.
Also, crabs, despite what you read here, are omnivores and do not just eat algae. Overcrowd on the crabs and all of your detritivore life (snails, ampiphds, worms, feather dusters, micro stars, even other crabs) is endangered!
Having stated that, I would not do a tank without crabs. They have an unmatched personality. I once had a half a dozen hermits, and then one day added one more. He killed the others. I was ****ed off, but I think my detritovre life is happy there is just one. I also have a single red Mithrax crab. he is not out much, but when he is, its quite a treat.
Chris