I'd suggest getting some buddies for your loner. Sometimes you'll find a particular fish doesn't care what color (or even species) his schooling partners are - for example, I have a Green Tiger Barb (notorious fin nipper and necessary schooler), who seems to have decided he's a Zebra Danio - he never nips anyone, and swims with the Danios quite happily. However, since this doesn't seem to be your case (though if he's new to your tank give him awhile longer for adjustment time), I think getting him some buddies would be a good idea. Tetras especially like to be in a school. Schooling fish will often become stressed when they're alone and sometimes even die, because they feel unprotected out in the open by themselves.
