Not sure if sexing can be done, I think most people buy 6 then let them pair off naturally.
However 21 litres is not big enough for one angel, let alone a pair.
no a 21L will not do. at least 29g is needed. and thats around 120L. u can tell the gender by looking at the papilla. the male's papilla is pointed, the female's is more blunt. this though is only evident when spawning is about to begin