Specifically, chromis, anthias, bannerfish do better in small groups (shoals). Many others, like tangs, schools as juveniles. Most are hostile towards conspecifics as adults, incl tangs.
Often you'll see like 5 yellow tangs in a 100g & it looks almost natural, but if u watch closely 1 is dominant and bullying the others, he just spreads around the butt-kicking. Its stressfull & not ideal for all fish involved.