2 females can be done.. I actually kept two females (siblings, though) in a 2.5 gallon tank, they got on fine. I definitely don't suggest it though.
Only females can be kept together safely, and they develop an alpha beta delta sort of pack.
The mating.. Well, its weird. I guess you could say that the females like it rough. I remember my couple chased each other and the female got very beaten up....However, on day three, they did it. A lot of things come into play when mating.
Pressure changes due to rain or storms often get them into the mood, as does a new water change and warmer water. It also depends on the fish's personality.
Basically, as soon as the female acts submissive, as long as the male isn't a fool, they'll do it.