I may be too late on this, but I would not recommend more than two females in a 20g. Your other option would be to get a pair. My females "guard" wherever they lay eggs, so that lets me know where the eggs are. However, if you constantly take out the eggs, the male will start trying to breed with the female once the eggs are gone. This can be very stressful on the female.
I have one pair of striped cons and two jellybean cons in a 55g. I've created lots of territories in the tank to keep the aggression to a minimum.
I also have a pair of striped in a 20g long, but my female had been by herself for a few months before I added the male, so I had to totally re-
aquascape the tank for her to accept him. She tried to kill the first male I put in there with her.
If you get your cons from the same tank, that would decrease the chances of aggression. If you have two females, eggs, even unfertilized eggs, still kicks off the aggression.
I hope this helps and please keep us posted.