They will sting anything, I have lost snails and hermit crabs to these guys. No fish yet though, they seem to know enough to swim just out of reach
I have mine in with my other anemones but I have them separated with rocks, the true anemones are on the other end of the tank
anything you do put in the tank with them should be at least 3" farther away then the longest tentacle can reach. You'd be amazed how much further thay can reach at night