Big hermits are great. I have kept them for years and despite being called not reef safe have never had one eat any of my soft corals(which is all I keep). I even keep small fish with them and they are way to slow to catch a fish. I don't even mess with the small hermits as the big ones do a way better job of moving substrate around. However I hope your rocks are stacked real good -- because they can move some heavy objects as they get bigger. Also more than any of your other fish watch your lunar and eel if you have one. The lunar will constantly pick at the hermit trying to bite chunks off and because they are so fast they can shoot in and get a chunk before the hermit can react. Main problem with eel is always stealing food from them and so they will starve if not fed every couple days a hunk of squid or shrimp as scavenging in your tank is not enough for them to survive. Enjoy him because he will truly be entertaining. My daughters favorite thing in the tank.