Thanks! I used a Nikon Coolpix 4800. It's a $200 point-and-shoot actually, lol. I was planning to get something nicer because it has trouble focusing in low-light situations, but everything else has been too expensive.
You'll know when there is a clear difference in thickness of the fish you have. females get real fat and males stay slimmer. depending on the species I think around 2" is old enough to breed. (ex. panda cory).
They might do it at a smaller size, but that was just to give you an idea.