I generally stay away from any non-live food when breeding fish, especially sensitive ones. Water quality is of paramount importance and live food makes retaining that much easier. Prepared food begins degrading the water the minute it enters the tank, whereas most live food can survive at least 12-24 hours in the tank, giving the fry a chance to eat them, before it dies and adds to the waste. Egg yolk is probably one of the messier, and therefore more problematic fry foods out there.
If you're looking to feed guppies, remember that they are a lot larger than many other fish fry, so the same tiny foods are not required. A lot of people have success just feeding crushed up flake food, which, unlike egg, has been designed to go to mush as slowly as possible. When I kept guppies I never fed anything special to the fry, and just stuck to feeding normal flake food once a day. While this won't result in optimal survival numbers for a breeder, I actually had much higher survival rates than I actually wanted or expected.