I may be misunderstanding this thread, so I'd like to clarify a couple things to make sure we're on the same page.
Substrate fish cannot manage on leftovers. They may or may not eat them, depending upon what the flake food is that gets to the bottom. I have seen some of my corys ignore flakes. These leftovers are best left for snails and bacteria to handle; by this I mean, don't expect the substrate fish to deal with it, but the small snails and then bacteria will, and that is essential for the biological balance in a planted tank. The snails break down organics faster into smaller bits that the bacteria can then more readily handle.
Substrate fish, be they corys, loaches, or other types of catfish should be fed a variety of sinking foods just as regularly as the upper fish get flake foods, and also a variety. I use 4 types of both alternating, and I would suggest no less than 3 different types of flake/pellet for the upper fish and 3 types of sinking foods. In both cases, one of the three should be vegetable-based, for better intenstinal health regardless of the fish. By different "types" I mean things like shrimp base, colour flake, algae-base, basic flake, etc. Omega One make many types of foods, and Hikari too. These two brands are said to be good, and in another thread someone mentioned New Life Spectrum I think it was. I do also use Nutrafin's sinking tablets as one of the sinking foods, simply because the loaches and corys seem to really like this.
Corys are primarily carnivorous, so they need "meat" such as crustaceans (shrimp), insect larvae, worms, or prepared foods that contain these along with fish meal. The vegetable food is not "natural" but a good nutritional additive and a prepared food with an algae/kelp/spirulina base serves this purpose. These foods usually have shrimp or fish meal in them as well, which is why non-veggie fish will usually eat them eagerly, thus getting their veggies "disguised" as it were.