I've had a H. diphreutes in my heavily "sps" dominated tank for a little while now with no problems so far. The 'bad (reef) rap" these guys get stem from the problems encountered with the very similar looking H. acuminatus, as they apparently are not reef safe at all and many LFS sell both speices interchangably. If you should decide to go hennie shopping and you have inverts and corals you value, I suggest you take along Scott Michael's book, Marine Fishes as it contains very good pictures of both species.
That's correct.
H. diphreutes is absolutely reef safe. My pair will pick at the rocks for pods, but that is all.
These are very fast growing fish however, so be sure you have a large tank.
Mine have doubled in size in 4 months and the largest is already 5" long. Within a year I expect these fish to be close to 8" long and 8" high.