If you want to build something to keep them from growing taller, research the word "scrog," or "screen of green" as it applies to plants. I don't see many people growing that way in these forums, but Google can help if you can't find that info here. It's building a screen of chicken wire or something similar to prevent them from getting tall. If I were you I would just let them grow some more, then cut the top. They will continue to get taller but they will bush out too.
About the sexing, this plant needs around 12 hours of light and 12 of darkness to begin to show sex. Once you start this light cycle don't stop it. Females will show white hair-like structures at the base of some branches, where the branch comes out of the trunk. Males will begin to show structures that have sacks of pollen, little balls, if you will.