My chromis school together but as far as fish go they are very low on the "cool" factor and lack personality. Chromis will slowly die off, or get picked off. I started a year and 2 months ago with 25, now I have 10 and have had that number for close to a yr. fish that school do that for protection, after some time in your tank they will become comfortable and stop schooling together anyway
I've always had die off with Chromis too... Always thought I was doing something wrong but I guess they just aren't as hardy as I've been told.