It is my experience that these tend to stay to themselves when there is a plentiful supply of algae to feed from. Although when my supply of bubble algae started to decline I noticed that the Emerald Crabs were dying off. I'm pretty sure they were not killed by anything else. They seem to be very secretive and perhaps did not venture out into more open areas where bubble algae would tend to grow. I think the emerald crabs were afraid because of the fish.
I think it depends on the size of the tank, how much rock, and how many fish you have on how well the crabs will do. It is also very important that if there is not a natural supply of algae available for the crabs you should feed with external algae into the system. I had to do this with my Turbo Snails for a short period because they are notorious for dying if not having enough to eat.
The Emerald crab should not mess with anything equal or larger in size if it is hungry. My only concern would be smaller fish.
Hope this helps.
T
