Sooner or later the channel cat will outgrow and eat the cichlids, then outgrow the tank. Buying "assorted" African cichlids is almost always a bad idea. They're typically auratus and zebra's, easily bred in FL ponds, and the most aggressive species for their size. Auratus in particular will claim the whole tank as their territory. Not a problem when they're small, but as they mature the killing will start. A 55 limits your stocking options for African cichlids, but you can keep several species of smaller/less aggressive mbuna after you get rid of the channel cat and sort out the "assorted" species you have now.