Well, I already satted I'd go with 6 female and 2 male guppies and then look forward to lots of fry. I you do any livebearers I would at the very least, given your tank size and the fact that you do not have it overstocked, go with a trio (one male and two females). A trio is generally suggested as males can be very... relentless in their pursuit of a little lovin'. This can often times stress the females out. Some folks like 3 females to one male as well. You could easily fit that into your tank as well.