LFS people are so full of BS sometimes... I remember being told I could keep an African cichlid in a five gallon, that goldfish do just GREAT in goldfish bowls, that no, mollies don't need salt, that you can keep a baseball glove sized frog in a five gallon all its life, that Chinese Algae Eaters are peaceful community fish that stay tiny all their lives, that I could keep an oscar in a 20 gallon, and many more such lies. There are a couple pet section people at a certain PetSmart that I do feel I can actually trust, but otherwise, don't believe everything they tell you.
Any way you look at it, it's generally a very, very bad idea to keep crabs--or crayfish, for that matter--with most any fish. If the crabs are small enough, your puffers will eat them. If your puffers are small enough, your crabs will eat them. If neither of them are small enough, they'll just tear eachother apart. It's kinda like pacman frogs. Keep two in one terrarium, you WILL, one way or another, end up with just one. Or a half of one. But not two. Never two. Same thing with fish and crabs, I guess.