seeing multiples of bright reef fish really brings that "wow" factor to reef tanks.
chromis IMO are the most hardy schooling fish, but they only "school" when they are small. after they get over 2inches they get territorial and do chromis-cide on each other. in a previous reef blue chromis were the "boys." but there are other colors of chromis, including some pricy ones.
anthias are my favorite shoaling fish. I have 10 bartlets, they all started female but turn male one buy one. they bring a hot pink color to the tank and referred to as the "girls" in my reef. i also have 3 sunburst anthias, and 4 dispar anthias. anthias arent as cheap as chromis and often referred to as "the gold fish of reefs". IME experience the key to keeping them is to add the whole shoal at once, then when 1 fish eats the others will follow. IME they learn to eat once a day, but until then an autofeeder can easily be used to feed 5-4-3-2-1x per day.
cardinals are another popular shooling fish, and tank bred cardinals can be purchased now. But as stated above they only school at night.
several species of gobies also school.
wrasses form harems like anthias. like anthias, females become sequentially male.
a few tang species can also be kept in multiples from small to juveniles. A group of blue or yellow tangs looks neat. Ive seen a tank full of baby achilles tangs- WOW! Dont call the tang police on me, Im clearly stating its a temporary thing.
HTH