The female will have a red belly once she is about 1 1/2 inches to 2 inches. The male will have a slight hump on the head. I started with 2 pr of convicts about 3 weeks ago, and put them in a 55 gal, and 1 now have 2 separate groups of fry swimming around. You want to have a place for the cons to lay their eggs, and it sound like you already have that. Just need to ensure you have a male and female. I don't think their would be a big issue with introducing another pair, but I wouldn't wait much longer.....In my tank, one pair owns one end, and the other pair owns the other. Another good item to use is a 6 inch clay garden pot, you can pick one up at a garden center for under 2 dollars usually, just rinse it out, before you put it in the tank...If you have any questions, feel free to send me a note, I just went thru, what your going thru
Icefisherman