I know things got a bit mixed up in the different threads, Ninja and conflicting advice was given, which is always unfortunate, but usually happens sooner or later on discussion lists.
I still stand by my position though, which is that although guppies can survive in a very small tank, it does not fulfil their needs for an active life. Though they are smaller than bettas, they are a lot more mobile; it's one of those areas where the one inch rule needs to be modified.
And as Tokis says you can't put a male and a female in a small tank because he'll just harrass her to death. It would be like being locked up in a broom cupboard with a sex maniac!