Chromis but be aware if you are not already about Uronema. This is real so need to quarantine. Some hobbyist report fight clubs and ending up with one. Others have kept groups (Parkers Reef for example, his display has several).
Depending on the size of your display Damsels may work but would need to pay attention to type and stocking order. Also note that they do not represent a true shoaling / schooling look. They hover around the reef structure and dart into their safety area when startled or chased. They can become, depending on the type, protective of their nest area if spawning. I have 11 Lyretail Damsels and they are a joy to have adding life to the reef structure.
Anthias already mentioned but they may or may not shoal/school. I have 4 pink square anthias and they sort of form a formation. Sometimes together with a distance between them and one off to the size. Sometimes they will group up. There are obviously other anthias like the Lyretail / Sea Golides but have some challenges.
This is such a difficult thing to replicate in our home aquariums. Group of Chromis during one of my dives in Fiji this past December.