Do you mean niger catfish? They are fast growing and very friendly. They will eat up all the food before the other fishes can get a chance. I had to get rid of mines for that reason.
The niger grows very fast. They are easy to grow and keep. They supposed to get up to 36". The red tail cats will get much bigger than the niger. You have to consider the heating cost when you are going to set up a pool to keep them in addition to the cost of running the filter system. It will be in the hundred every month.
I've raised three Niger Cats. The first couple died at 16" (power failure) and 22" (fire in the garage). The third one I raised lives in the 12,000 gallon "Flooded Forest" tank at the Oregon Zoo and is well over two feet long and massive. Like everyone has said, they grow pretty fast and are pigs at feeding time. They are very easy-going with tankmates, but they can accidentally cause damage with their scutes, especially when they get big.