Kept singularly is fine, but be warned that like all fish they have differing personalities. You might get an overly aggressive/boisterous fish, or an excessively shy/timid one. They're generally peaceful fish though, and since the tank will already have fish in it, it shouldn't have aggression issues like if it were solo and think it owned the whole tank.
Since you have angels, don't get neon tetras as they will become food for them. No smaller shoaling fish than rummy nose tetras.