Especially in a newly established tank, cories wont be able to get enough food just from leftovers from the other fish. I have sinking shrimp pellets that my cories eat, and they love them. They also catch a couple of flakes that land at the bottom, and whenever freeze-dried bloodworms make it down to them they eat too. They also enjoy the weekly mini algae disc that they all share.
With the sinking shrimp pellets, I drop down about 5 of them, and my cories munch on them through out the day...and I only end up adding sinking food once a day, even though they feed continuously.
For my top fish, tetras (for the moment that is all else that is in there), I feed them twice a day (they are more than half way grown).
In my smaller tank, I feed them twice a day as well, more in the morning than in the evening. Their evening is more of a snack. I am careful not to overfeed this tank since it is so small and I dont want a spike in ammonia...in a small tank, the littest spike can be bad.
In my larger tank (29 gal) I dont worry about it too much, all the food gets ate, and even if it doesnt, I dont ever get a spike in ammonia (it has been fully cycled for a while and I havent had any issues with the ammonia except for a mini re-cycle it when through when I put a new filter on it, but it was a very very tiny ammonia spike, it was something like 0.025 and was immediately addressed.)
Congrats on finally getting fish!! I know how exciting that can be!! And welcome to TFK!!!