I know it depends on the fish, but my Angelfish (queen) ate leathers and paly's on me constantly. I bought her from someone that swore up, down and sideways that she'd be fine. That was basically the end of my adventure with non-reef safe fish.
I'm under the impression that most are not reef safe due to them eating inverts rather than coral, with exception of angelfish and a few others that aren't completely coming to mind right now.