I've raised frogs from eggs, but never fish. There are probably some parallels though. Tadpoles live off of their yolk sacs for about 7 days...in their case, you can tell when they're ready to start eating because they start rhythmically opening and closing their mouths. As for how much of the food...I can only assume that, like tadpoles, giving too much is way worse than slightly underfeeding (clogs the gills). In the case of the tadpoles, the food (a slurry) clouds the water a bit, and as the babies eat it up, the water clears. The goal for the taddies was to have them clear the water in about 5 minutes.
Another thing I learned with the tadpoles (they were African Clawed Frogs, by the way) was that the filter set up is really important. The waterfall type filter I had caused way too much turbulance. I bought a foam filter instead, and it worked very well. That would also help from food clogging their gills if you get a little too much in there.
I don't know if any of that helps or not, but good luck!