I believe the minimum for cory cats is like 6, and for a good reason!
I only have one in my tank right now because the others died off... It was really weird. But hes going strong. It makes me sad though, I never see him and when I do he only darts out to get food really quickly then back into his hiding spot. When I had 6, they were all over my tank all the time. It was awesome, but now I have a new 20 gallon for him and his future buddies with nice black sand in it :)
Thats another thing, did you research about cories and sand?
I know people keep them in gravel and say its ok, heck thats what I'm doing. But they really fair better in sand. My little guys barbells are so messed up :( and my gravel is smooth.