well there are so many different kinds in the Salt world, but mostly they are just scavengers and won't hurt anything that can swim or run away. I think there are types that are not good for corals and such though, but I am not an expert. In general you should have at least 1 extra shell that is slightly bigger than the one your hermit is currently carrying around. If you have more than one hermit, they may fight with each other and steal each other's homes if there are not extras laying around. Plus as they grow they want bigger homes!
Hope I helped