Beautiful little guys you have there, i have always been very fond of bronze cories but the soze and bio load is what put me off as I have a hpabit of choosing smal, less bio losd fish, such as wild strain guppies, pygmy cories, neon tetras, sparkling gouramis, etc,
If I were you I would most likely return them, as Corydoras species..... Even the somewhat larger ones prefer sand and use sand to sift through. Most bottom dwellers actually do prefer sand, there are a few hard bodied exceptions though such as Plecos. But Corydoras have somewhat soft bellies and with constant irritation on them, fungus accumulating is very common, as well as there barbs deteriorating. I would not recommend them in gravel tanks unless the gravel is fairly small such as 1-2 millimeters in diameter,
Also. What do you feed them? I'm assuming that you feed them meatier foods such as shrimp pellets, right? They are mainly carnivorous fish with the acception of eating algae wafers on occasion, they do prefer meatier foods and this is what I would recommend, mine love blood worms mainly but as you know, this is not a staple diet so I would recommend a diet of about 30% shrimp pellets 50% catfish pellets 10% algae wafers 10% bloodworms, brine shrimp, or California black worms
The gender of bronze cories are easily determined, I usually know the gender by the plumpness of the fish. Mainly females are slightly plumper, and males are thinner. Females are especially plumper when filled with eggs