I have several dwarfs
1. Black Nox
2. 1/2 Black
3. Bi-color
4. Flame
5. Potters
6. Lemonpeel
Every once in a while, one of them will pick at my corals with no detrimental effect. They are fed well, and, the irritation is minimal. I have a 210 reef with ricordea, softies, and, most notably many sps including acro's and monti's.
In fact, they seem quite beneficial. Recently, I created a frag tank. Naively, I did not and a clean up/algae control crew in this 40 gallon breeder frag tank. A few of the frags developed a slight case of hair algae. When I placed one of the frags back in the main reef, the drawrfs had the algae completely removed in about one hour. The four tangs present, did not even go near it. I support dwarf angels, and, if I get more courageous, I would love to add a regal and an imperator and a queen. it probably will never happen, though.
Andy