Whether they get eaten in the main tank or not will depend on lots of factors, like who is in the tank, the personality of the parents, how heavily planted the tank is and how bright they are (survival of the fittest).
I had this idea that keeping my platy fry in the main tank would ensure that they got eaten. It seemed to work at first, but now I have more plants and several batches of fry of different sizes which seems to confuse the adults so they don't know what they can eat and give up. We had a thread about this not long ago, some posters said their main tanks get overrun with livebearers, others can't keep any fry at all.
I do remember years ago bringing up black molly fry in the main tank, which was hardly planted at all, must have had a lot of daft fish in there. But this won't work for everyone.
As for food, I fed my first platies powdered baby food by Tetra which they took from the start. The second batch were born while I was on holiday and seem to have fed themselves. Haven't bothered feeding batch no 3 and they look in fine condition. But then mine is a planted tank which has been running for 6 months so probably full of infusoria. If yours is not I would have thought molly fry would be fine with any baby food from the shop or even failing that very finely powdered ordinary flakes. Egg yolk is fine but messy. You can make your own infusoria by putting hot water and a lettuce leaf in a glass in a sunny window; I did when I had molly fry years ago.
They will soon graduate to ordinary food: I saw a 2 day old platy fry tackle a tropical flake yesterday!
