Got my first Apistos, now questions

[BillV]

So I bought two Apistogramma Borellii blue, a couple of weeks ago. It took a good week for them to really start exploring the aquarium. They are small, like an inch long. I have no idea of sex, I am just beginning to see ever so faint yellow around their heads. How big do they need to be to sex, when does their color come in? Also, behaviorally they've hung around each other a lot. This morning I noticed aggressive behavior towards eachother for the first time. I have a cichlid stone in the tank, along with driftwood, rock and a lot of plants. This morning when i turned on the lights they both were in the stone together, then they started flaring gills at each other and charging each other. Is this territorial behavior a make/male, female/female or male/female thing? Will they be establishing different territories now, there are plenty of spaces to do so. If i can figure out how to post a pic i will show the tank and try to show them. But they are small and drab still. Also, could i add more of them, or a different type, its a 29 gallon tank, with 4 sterbai cories and emperor tetras and cardinals. The apistos are smaller than the tetras