I have 4 platies along with a few other fish in my 110litre tanks and we've basically let nature take it's course. There are loads of places for the fry to hide and feel safe, including a cave, live plants and bogwood. So far we have 2 midsized fry and 8 teeny fry all running around. They are that secure with their hiding places that the have been seen going all the way to the surface and also to the front of the tank!
It depends on what you're trying to achieve tho, if you're intent on breeding them and keeping them leave them in the breeder net. But you may find you're over run b4 long... As for food, while they are teeny tiny you can get some Liqui-Fry stuff which is food that disolves into the water and is small enough for them to eat. After that ground up flakes should be fine.