I had 10 in a 75g for 2 years, along with a couple other fish. I only put them in there as "dither" fish, expecting them to die off slowly.
About 6 months ago I had to "fish" half of them out with a hook and line, as they had gotten too big and were making the bioload in the tank much higher than I had intended.
That doesnt seem to be the normal story with blue green chromis though, most people seem to experience them picking each other off. I'm not sure why I didnt, I had the basic average noobie setup, a mixed LPS/SPS reef with a flame angel, a lyretail anthias, a clown pair and the chromis, all in a 75g. They did great, never lost a chromi for 2 years straight.