I know that one will be depressed, two will fight, but is it normal for three or more to establish a pecking order? I've heard of it in smaller schooling fish but haven't heard anything about it relating to catfish.
Yes even schooling catfish will sort out a pecking order between themselves, usually the largest fish will take the head position and will keep smaller fish in line with occasional nips and bumps.
My 5 seem to have a pecking order....They dont school together and 2 of the larger ones are always by themselves..And when a smaller one gets close he'll charge and they seem to tussle "spin around" quickly....They never hurt each other and once you get used to it , it's actually funny