The deal with chromis [and most fish] is that they do not school well unless under threat. IE, if you have a nasty blue velvet damsel to rush about and nip stragglers, they will school everywhere.
Schooling is more of an 'us against it' response. If you want a pretty blue fish to flit back and forth, a single chromis will do it and leave everyone else alone.
Chromis will attack each other and reduce numbers until they're comfortable with their environment size. This, in a smaller tank, is about one, maybe 3. Seven, I fear, is going to see some attrition. I love them, but only the dominant tends to have the really high color, so one is probably the best answer, IMHO. It's what I have.