Ok, last time! Having 3-4 nickel sized angels are fine in a 20 (high or long). I breed angels and thats what I use for adult angels to breed. Angels will pair off relatively quick but they have to be mature. Angels will become adults before they become large. Having said that, you will want a larger tank eventually. Yours could be too small, no pics hard to tell. Sexing angels is very tough to do. People talk about the hump-head thing, but that could just be in the strands vs actual sexing. I have not found any actual information confirming these claims. You have to wait until they pair off to REALLY determine the sex. The bigger question you should be asking yourself is in the event of eggs and then hatching, do you have a much larger tank for them? Breeding angels is easy, its raising the fry that becomes very difficult. A tank of at least 55 gal I would recommend for raising them. This is just a brief statement of my recommendations. Good Luck.