you're in the US, so your friends should know about starlings. starlings are those little black birds that fly around in groups of a hundred birds. if one starling ever flies away from a spot, all of the other starlings fly off too. you'll never see one starling by itself, unless it is physically unable to keep up with the rest.
cory cats and other schooling fish are just like starlings. while you technically *can* keep one by itself, it will feel very insecure and won't act the right way. it will be more likely to hide and it won't eat as well. most schooling fish will also look very washed-out and pale if kept in too small of a group. just like it would be cruel to keep a starling all by itself in a tiny cage, it isn't nice to keep a single cory cat in a tiny tank.
that's why you *shouldn't* keep only one or two cory cats.
Starlings "here in the UK we only see them in large flocks when they are ready to migrate or fledging juvinilles" thank god lol, as for fish alot of it is safety in numbers issue, large shoals of tetras will split up and investigate the surroundings as individuals but as soon as something spooks them they quickly shoal (ive found a tank where the fish constantly shoal is an insecure enviroment), my mate who breeds discus swears by tetras as it kinda gives his discus a feeling that all is well when they are active, cory's are much the same as they may pair off or hang out in small groups but when one small group meets another they go wild, Anyways why would you not want these fish in groups they are fantastic to watch.#
As for keeping 8 pandas together etc i dont think it matters all my cory's will hang out together at some point regardless if they are panda's, bronze, peppered, albino etc they just love interaction as pointed out above..
