Most fish food out there isn't made for a 2 foot fish and would cost a small fortune to keep a large fish fed. Also, with the limited selection of food for large fish, there wouldn't be very much variety if you stuck strictly to fish food. So, yes, I do mix some cat food into the rotation along with market shrimp, beefheart, baby carrots, and fishfood.
One thing to watch out for is that most catfoods aren't very nutritous and use sawdust as a main ingredent. This gives the animal a sense of being full without the nutritional values. Make sure you pay attention to ingredients. From my experience, I have found that the better catfood dosen't float.