That's a much better idea, the catfish would be a lot happier in a tank that size. Just for future reference, do as much research as you can before you buy, that way you don't have to make those decisions. A lot of times you will get conflicting opinions on different fish, but do more research and trust the experts. There are people on this forum who have been keeping fish for decades, most people in the pet stores will, unfortunately, tell you what you want to hear so you buy their fish. Just remember, you can never do too much research
Good luck with your tank.