I have bad news

. Channel Catfish get several feet long and shouldn't be kept in aquariums IMO (and shouldn't be sold at LFS either). It will get huge and will eventually eat your other fish that it can fit in its mouth. I'd take it back.
Otherwise, you are on the right track.

Check your water daily and do water changes when the ammonia or nitrites are above 1ppm.
Also, since the Africans you already had are Mbuna (and I'm assuming the new ones are too), they should be fed a mostly plant based diet. I'd pick up some food that is Spirulina based for their main food. Too much protein (like brine shrimp) can cause Malawi Bloat, which isn't good. Pick up some Spirulina based flake or pellets at the LFS when you take the catfish back.