[Sk8r]

Yep, crabs are iffy. The only crabs you really can sort of almost trust: the micro hermits, the scarlets and extreme micros that never get larger than a cerith shell---well, I had one scarlet that took to wearing a small conch shell. They're much better behaved re snails. But any pointy shell snail that falls over in a tank full of hermits is probably toast.
2) the emerald mithrax crab---useless, ime, but kind of cute. I suspect I have had them nip fish fins, but never could prove it.
3) very specialized and fairly uncommon acropora crabs.
4) as far as I know, some of the little filter-feeder crabs which are rarely offered in the trade. Can't see how they would ever be a problem.

Anything else---crabs ultimately start noshing on ANYTHING. They are nature's little generalists, and if they are large enough to get it in their claws, it's toast. [I still love my micro hermits, Cap'n. Lol! But I know they're thieves and scoundrels, and I *hate* it when they grab stuff from my corals.]