Porcupine Puffer teeth do not overgrow like many other puffers such as Dogface or Green Spotted Puffer or any of the freshwater puffers (whose teeth are the fastest growing I find). They are a little bit different. Anyways, my brother has had his Porcupine Puffer with mushrooms, leathers, gsp, macro algae, xenia, and stuff like that for 7 years now...no problems. I also have mine with that kind of stuff and he could care less. Now anything hard that goes in the tank is a different story. It will most likely be destroyed. Someone mentioned tobies being better but in my experience they are much worse. The bigger puffers tend to nip less.
Its always a risk with puffers and anything from corals to tankmates but you can get lucky. I would never add one to a nice established reef though...just start with stuff like mushrooms and leathers and see if he accepts them and go from there.
As for crabs, mine rarely touches them. There are hermits that have lived with him for over a year now. He has eaten the odd one but I just replace them once in a while. He usually just swims past them though and leaves them alone. But again, I got lucky with mine
The only other problem is that they need a tank much bigger than 65G if thats what you were thinking. They reach about 10" in home aquariums but its a BIG 10" when you consider thickness.
Another thing is adding small tankmates after the puffer can sometimes be tricky...not usually but you do have to be careful. Mine ate three chromis I added to the tank a while ago because he thought they were food. You usually want to add them with a net and guide them down lower in the tank. As soon as these fish hit the water, he started chomping them!!! It was a bloodbath...
Cute fish with sooooo much personality though. Mine:
