I have a baby Potters in a 12 AP reef, and he doesnt nip at anything, he was kept in a reef at my LFS. Hasn't touched my elegance, various zoa's, mushrooms and frogspawn. He picks around the corals but not at them, he is not taking chunks of flesh out of them, but simply removing pieces of algae growth on their skeletons.
I have had very good luck with Flames, Potters, Coral Beauties, and Atlantic Pygmy
Like everyone is saying however, no matter what kind of dwarf there is still the possiblility, and nothing is to say that one day he wont go and eat all my coral. I am ready to tear my tank apart to remove him and its just something to keep in mind.
Some angels have a better track record than others, with the worst ones being below (at least in my experience and dealing with customers)
listed in order of worst to best in terms least likely to be reef safe
Lemonpeel
BiColor
Black Nox
Ebli
half black
Good luck, and hope to get lucky, remember ther is always a chance that a good angel will go bad...
BTW has anyone else been noticing lately, that peoples Blue Hippo Tangs are becoming less and less reefsafe, with more threads on them eating zoas and corals?