I had 5 green chromis in a 150 with quite a bit of LR and hiding spots, but after 9 months, only one remained. That chromis killed off the other 4, and now lives happily with a pair of clowns, a blenny, and a goby in my 65.
This is just my experience, as I've read many other successful stories of reefers keeping groups of chromis in their tanks.