Just to back up the statements of others, your LFS guy is wrong. If you want happy cories you need to buy them in groups of at least 4, preferably more. Also, sand really is the best substrate for these guys. Buy a 50lb sack of quikrete play sand from Home Depot for $5. The cories will be MUCH happier.
I had a lone swartzii cory for a few months in a tank with rough gravel substrate. I always worried about the guy, his whiskers went away, he never seemed to eat, and he looked generally unhappy.
I started a second tank with play sand substrate and tossed him in there with 4 more swartziis, and it's like he is a new fish. They all seem to have a great time sifting through the sand for food. He's only been in there for 3 weeks, but I fully expect his whiskers to grow back. Cories are so much more fun to watch in groups too. They'll all sit in a line at the bottom of the tank under a rock when they're resting. When they're active they're always sniffing around in a big group. You really should not keep these guys alone.[/list]