Teri:
I don't think you're undergunned for that size catfish with a 7 wgt. An 8 wgt might be a better choice, but you have to go with what you have. An intermediate or a full sinking line, and for cats that size I would use a straight 20 lb fluorocarbon leader, 6 feet long for streamers and 4 feet long for a floating fly. The heavier tippet will allow you to put more pressure on the fish with the rod and since this isn't finesse fishing you don't need a tapered leader.