Angelfish can be finicky eaters when they are new to an aquarium or uncomfortable. Is he/she to the aquarium? If so, be patient and continue feeding small amounts regularily, it should eventually come around. Since you have other fish in the tank you shouldn't have water contamination problems from leftovers. If you suspect disease, describe the symptoms, general appearance, coloring, swimming pattern, etc.
Sexing angels is impossible when they are young and even when they are older sexing can be tricky. When older males have larger heads with a characteristic 'lump' developing in front of the dorsal fin. The best way to sex angelfish is to actually see them breed - as in who lays the eggs and who fertilizes them. A very good bet is about two to three days prior to mating, the female's spawning tube will descend, it is thicker than the male's, whereas the male's tube descends withing one day to hours before spawn. Good luck with your angel!
Fred
