I have had both the scarlet and blue hermit crabs in my 65 for around 8 months with very little problems. All seem to get along fine don't even bother the snails. Occaisionally I will drop some cheap pelletized food on the vottom(once a week) And you can watch the crabs run and chow down. All the other times you watch they are just picking thru the rocks and sustrate cleaning up as required. My biggest complaint is cleaning out all the shells that are shed every 4-6 weeks,seems like they all doit around the full moonI think if you make sure they have food and other shells to move to you reduce the amount of canibalistic attacks.
Mike