Chromis are pretty hardy I added 1 , 2 weeks into my cycling process And he has gone through some anomie peaks ( up to .50ppm ) but he is fine , same with my other 2 , and they have to deal with my Clarkie clown . Did you have a chance to watch these chromis before you got them? I usually window shop my fish for a few days at the LFS
to make sure they are feeding and swimming well and no visible bugs . Also all 4 of my chromis are so timid they scoot away from my cleaner shrimp.
I think you got some fish that had one fin in the grave already :-/
And no I didn't QT my chromis either :-/ , I am working on a QT tank soon