Hi, Lisa: I haven't kept one of these, but I did have a Tiger Shovelnose for a couple of years. Really sweet fish, but past a certain point, tough to justify keeping at home. He now resides in the Tennessee Aquarium in Chattanooga. Red-Tails if anything are even more likely to eat their tank-mates (The curator at the Aquarium told me that their previous TSN had been eaten by a Red-Tail). They'll even eat turtles!
Younger (smaller) ones seem to co-exist well with fish to large to swallow, though that is a moving target. If you start with a small one, large cichlids should be OK for a couple of years, though I'd shy away from pike-shaped varieties. Too easy to swallow. Give your catfish someplace to take shelter till he gets his growth. Spawning or grouchy cichlids can and will persecute them from time to time. I kept a roof tile for my guy to hide under till he got big enough to feel confident and comfortable with his co-tenants.
Red-tails are gorgeous fish, though! I'm not sure what I envy more, the catfish or the 8-foot tank!
Here's a pic of a 3-foot RTC with an 18-inch clown knife fish:
http://www.aquamojo.com/Redtail.html