RTN doesn't "spread" so I think you're safe there. We need to know what your alk, calcium, and phosphate looks like. Big thing with SPS is they DO NOT like alk swings. Alk swings can cause STN/RTN if the swing is big enough. I also loss a few pieces when I dropped my alk too quickly.
Also, Coral beauty is a type of angel fish, and they are known to nip at corals, which can / will kill the coral. Imo, take him out, especially if you see it nip at corals as you mentioned.
Also, given your tank and the age, it still seems pretty new and not well established yet. While some people have success with SPS right after the cycle, not everyone is able to do the same. I would also assume the tank is too new for sps.