None of the ones you will get on live rock will grow any larger than you see there, and they are good scavengers. the ones that eat fish are the ones you see at the LFS
for sale. The large green ones.
As for the food, you should have made them feed it in front of you before buying it. They do tend to tell stories here and there. That said, go to the LFS
and pick up some live black worms or blood worms if he's not eating. Mine ate only those for a couple weeks, but never went to flake. I've had him a year and he still turns his nose up to flake and pellets. I feed him frozen chopped fish and shrimp mostly, along with nori on a clip of course.