Another thing to consider is the needs of the crabs. True, fully aquatic freshwater crabs are rare. Most, including all that I have seen in the stores, must have surface access. They are escape artists, as well, but will die if not provided with this in the form of a floating platform, sturdy plants that break the surface, or a paludarium style setup. They typically live only 6-12 months in most tanks, simply because they do not have a reliable means of getting out of the water, so resort to climbing cords, and from the cords, they end up under furniture, dead.
They are cool, but combined with their tendency to grab fish (mostly at night, they will get even top swimmers this way), it's better to have a setup just for them, rather than mixing them in a community setup.