All clowns start out as adolescents. The largest will become a female, and the next largest a male. Maroons will most likely kill all others outside of the pair. The recommendation for pairing is to get a second clown that is much smaller than the first, and they might eventually form a pair after some pretty intense aggression. In my first attempt to make a maroon pair, one candidate was killed. They were both the same size and added at the same time. When I added another maroon, I added a slightly bigger one (it wasn't as big as I thought) and it seems the new one has become the dominant one. My clowns were small to begin with so there was no chance of finder a smaller clown that the one I originally had. The aggression wasn't as bad as I thought it would be, as the first resident started to wiggle in fear about a week or two after adding the second one.
Clowns that live alone will probably develop into a female if alone long enough. No I don't know what that time period is. In addition, clowns will not morph from a female back into a male. It is a one way train: adolescent -> male -> female. So once a female always female. If the female dies the male will move up and change to a female. I think I read about some rare instance in which a female morphed back into a male but this is not the norm.
This should pretty much summarize what everyone else is trying to say.