Adding a couple more (to have five minimum in total) would be advisable, but not until you are certain they will be OK. Most aquarium gravel is not sharp, though some can be. I had corys, dozens of them and wild caught too, over fine gravel for many years. But now that I have sand I have seen a difference in their behaviours, and I certainly recommend sand with corys. This too has to be smooth, like play sand.
As for food, offer sinking foods in a variety of 2-3 or so. One should be a veggie-based food, like Omega One's veggie rounds or similar. The shrimp pellets are good nutrition too. Although not vegetarian, the veggie food is a good idea for their health, and there is some "meat" in these too. Corys will not eat algae per say, but they may graze on it for other microscopic foods. They cannot survive on "leftovers" but must have specific foods like those I've mentioned.
BTW, the "julii" corys are likely not Corydoras julii, but probably C. trilineatus. This mis-naming is widespread. Doesn't matter, the fish are very similar and have no contrary needs.
Byron.
