My cories are doing well on my gravel-like plant substrate, no missing barbels or scratches on their bellies. I don't have sand in there because my betta would eat it and get himself sick.
I really would recommend not keeping one by himself. I had a big pH swing in my tank shortly after it cycled and I lost all but my one peppered cory. He did fine by himself for a few days while I worked to get the tank to where it was safe to add more, but after about 3 days of being alone, he started showing signs of stress, swimming up and down walls and running into things. It was a bit of a fiasco finding new tank mates for him, as the of the first 2 I brought in, one had septicemia and the other had a swim bladder disorder(fairly common in cories, sometimes an early indicator of septicemia, I've been told). He was just happy to have buddies in his tank for about a day, even if they wouldn't school with him. I brought those two back and had to let my tank sit for 2 more days while I made sure that no one had caught the septicemia. By the end of the two days alone I thought he was going to bash himself dead against the heater. He settled down when I was finally able to add one friend, but neither of them really looked adjusted until I added my third.
They are a social fish. Even if he looks like he's doing ok, at the very least his immune system is compromised and he will be extremely liable to get sick and possibly spread it to your betta.
Proud mom of Samuel, the red, white and blue VT and Darius, the purple and red HM.
Samuel's Tank Mates:
4 peppered Cory Cats
1 Siamese Algae Eater