This is a good question, in my experience they begin to pair up just about the time the colors enable/ suggest sexual distinction. I agree with Jason, color approximates puberty. At 5 inches i am guessing they are about 9 months to a year old. In my experience they pair bond first but dont actually spawn right away. I think your probably right I would expect a compatible pair to start acting like what you have noticed. I have not had any actually spawn under 6 inches for what ever thats worth, since there are other factors involved as you know.
Enjoy them