I would need to check further to track down articles, but various pygmy Corydoras are involved in schooling behaviour with tetras and other fishes. If I recall correctly, the various unrelated species have similar colour patterns, so at a glance look like a school of a single species. This apparently also allows for rare species to gain the benefits of schooling, without ever being present in large numbers. I don't know if embers are ever involved, but I do recall seeing a species of Cheirodon swimming among a shipment of Corydoras hastatus is an aquarium store. The two species had almost identical markings and were presumably collected together.
Tony