You can feed them anything small enough for them to eat. I feed my newborn poeciliids on crushed flake foods but special fry foods are also made for them. Another good growth food for newborn livebearers is newly hatched brine shrimp. You can get an easy to use brine shrimp hatcher or DIY one yourself. One of the advantages of feeding BBS is that they move and act like food to the fry.
You should not be losing that many fry. Have you checked the water quality? Fry are an amazingly large biological load for your filter to deal with, given their extremely small size. A water change each day would go a long way to getting higher survival rates.