I buy and sell lots of livebearers in pairs or sometimes trios at auctions. If you are serious about caring for your fish and take the time to separate gravid females to let them give birth in peace, it can work quite well with a single female. On the other hand, a large breeding colony will almost always have about equal numbers of males and females, that also works out well. The only time you worry much about the ratio is when you have a few but not very many of a species. If I am trying to keep just 6 or 8 of some species, I always try to make the ratio heavy in females. Since many fish keepers do not keep the large colonies that I do and are not simply trying to breed a selected pair of fish, the general rule works out well for them.