I'd advise against it. They are not fully freshwater and do need some salt in the water. They also need land as the others have mentioned. Not to mention if you are planning on a pleco, plecos+ salt and/or Plecos+crabs = disaster. They do also need an excape proof hood as not only can they climb up plants, wood, rock, filters, airline hoses, etc..., but they can scurry right up the silicone corners! And you woulden't believe the tiny spaces these guys can get through!
All in all, crabs are awesome to have, I definitly want to have some again, but they really are better off in a tank made specifically for them, be it only them, or them and a few fish that you don't mind them catching (and snacking on) now and then.