As mentioned, it depends mostly on the particular fish, I have seen two coral beautys rid tanks of green star polyps, and heard several other stories of pygmys eating small polyps. Up until recently I have never personally kept a pygmy, but I have always wanted a certain angel, an african flame back, fine fish
Anyway, a friend is breaking his tank down and I couldnt resist adopting his flame back, it will eventually be housed in a 29 gallon tank dominated by LPS corals so there will be no small polyps for it to pick at, but at the moment its in my frag tank, and it defintely has a taste for Acropora polyps
I think 50/50 is way to good of odds, IMO your taking a risk adding any angel to a reef tank containing corals with small polyps, sps, or soft corals. HTH