A lot of social fish don't hang out tightly in tanks without scary predators. If I look at my tank now my shoal of embers is as spread out as my otos. The otos will often swim together early mornings, or late evenings. There are days they spend a lot of time going up and down together - often with my habrosus corys in tow. They definitely give me the impression they notice there are fish of the same species and interact with them deliberately unlike some single fish that only cross paths accidentally. For a proper shoal I think you'd need a bit tank and a lot of them.