I've got two sets (12 in total) of Platy fry roughly a month old. They spent their first 4 weeks in a breeder box and 2 days ago i transferred them into their own 20L tank and they're doing great, the logic behind it being they'd grow quicker with more room and i can therefore re-home them quicker. I've also got a shrimp in there to clean up after them.
I fed my finely powdered fish food twice a day until they were slightly bigger (about 2 weeks) and could eat bloodworms. I give them bloodworms every second day now and flakes on the other days, I don't chop the bloodworms up and they manage just fine.
Good luck with your fry. This is the first fry i've had and it has been fascinating and rewarding watching them grow.