I put frags of shrooms, xenia and other hardy corals that have overgrown my nano and would otherwise be thrown away, in with my GSPs. They appear to be OK -- they can survive the occasional nip. That said, I wouldn't recommend it - and it would be irresponsible IMO to put anything wild-caught in with them.
There's also a large crab that hitched in on the live rock, and it has survived in with them for over a year - quite a feat considering how big and hungry the three GSPs are.