I had a bit of the opposite problem. I bought 6 cardinal tetras that were all from the same tank so I thought they'd all be buddy-buddy. The largest tetra had a slightly more turquoise stripe than the rest so he got ostracized. They picked on him, wouldn't let him swim with the group, and nipped at his fins a lot (which earned him the name 'Stumpy').
So he ended up always swimming alone by himself and all the others schooled together. :<