I have a 10LTR tank that I am using as a hatchery for Platty fry. I can have 6 medium sized fish in this once fry are old enough for main aquarium. Think you should be ok with the number you have. The main problem with over stocking is fish waste causing ammonia problems but as long as your fish...
Are you sure you want to set up again? I had 2X 4ft tanks many years ago and have just set up a 40ltr tank again. Begining to wish I hadn't due to things being far more expensive this time round and I don't remember it being so much trouble.
I'm rearing young Platy fry in a 10ltr tank at the moment and feeding Liquifry No2 for livebearers. In a communal tank other fish will eat the young. Either use a hatchery that you can sucker to the glass of the tank to separate pregnant female until she gives birth, or a floating hatchery...