The reason for having 2 females to one male platy is because with just one female the male will harrass her to no end and she'll get pretty stressed out. Bettas are pretty unique in the 'no other males' respect, but even with them if males grow up together they get along in the same tank fine. Your black neons may be females but they just might not be producing eggs right now, due to diet, age, water conditions or any other number of things.