I'd also take a chalk bass over a chromis any day of the week...
though they tend to suffer from the same hierarchal? aggression that the chromis do they have a nicer disposition overall and are much more attractive IMO.

My experiance. I have had my 3 blue chromis for 1.5years now. They have always schooled together, they do pick on each other but more a higharchy kind of thing more than aggression. The odd part about mine has been growth, there is the big one, the medium one, and the runt. They were all almost the same size when I got them, and are now all very noticeably different in sizes.

I vote 3, I had three in my 55g, and in my 120 it seems empty with only three, but I would create chaos adding more now in a established tank with settled fish.

I have 5 green chromis in my 33Gallon Aquarium.
I have had them for about 3 months now, and they are all fine. I put them in all at once though, that might of made a difference.
They occasionally school when my dwarf lion fish tries stalking them, but hes to small to eat them.
The chromis take turns chasing each other around though, but it dosnt strike me as agressive.

Before I had the chromis I have school of 5 glass cardinals and they got into mortal fish kombat untill one one was left, and then that one got eaten by my anemone.