WELCOME TO AA!!!! 
I have not had any problems with the crabs they have been very peaceful and pretty much do thier own thing they are especially active at night. I did have to to some rearranging as they would climb up the plants and climb into the filter where they were happy hanging out but I didn't like them there. They never ate the plants and were pretty good with cleaning the bottom of the tank, they are VERY cute when they eat using thier little pinchers. In my research I found that the crabs were better with other fish than say freshwater lobsters, crayfish and shrimp, just make sure the crabs have places to hide! Right now while my tank is stablizing better we have them in with my boyfriends HUGE cichlid ( around 8 inches long) and a huge platy (10 inches long) and a clown loach and they are fine with those big fish as well
I had a FW crab in a small tank (?20gal?) once. Then one evening, to my horror, i glanced over at the tank and saw the crab which had climbed up a decoration, leap off onto a small dwarf ciclid, and begin to drag the struggling fish behind the decorations. The case of the disappearing fish in this tank was now solved! I intervened, saved the fish, and got rid of the crab. What was surprising was that the fish was bigger than the crab, and the crab had the fish gripped with its walking legs, not its claw. Perhaps my tank was too small, but I suspect that the crab will prey on fish in larger tanks too. The fish have to be much bigger than the crab to prevent this.
