They're just plain old river crayfish, the importers put them in with the goldfish to eat the poop. The claws will get much bigger, they're still growing back. (they remove the claws so they dont poke the goldfish in transit)
They will eat everything though once their claws are fully functional again (they look 2 molts away from being full again), including fish, snails, shrimp, other crays, plants if they're hungry enough, lol.
I had a few for a while, I adopted them from petsmart too. Mine got to about 5 inches before I had to get rid of them, and careful, when they get that big their little pinchers aren't so cute.