Here's my experience with Emeralds and other Mithrax crabs. I have kept them for years in my different tanks. Get small females, they are much safer. You can tell because they are much smaller and have smaller claws proportionately. Once the males start getting large, they get more aggressive and will go after other males. I had one large male in my 90 that would routinely knock over all my rockwork. Some of the rocks were 25-30 lbs each so they are pretty strong. I have also had a male just start chomping down on a frogspawn years ago. Basically, once the males get a carapice bigger than a quarter or so, they start getting a bad additude. I have never had one problem with females though. They are great algae eaters and mow down just about anything. I usually do keep one small male though because then the females have babies and the corals love that.
If you want to be really safe, you can just get a bunch of tricolor hermits. I've never seen them go after anything and they stay pretty small.