Blue-green make the best dither fish in the hobby. I've always had one or two in my QT's to 'teach' new fish how to eat, and have zero aggression to newcomers. I've probably forgetten all the fish, my Naso, 3 Butterflies, multiple angels, have 'learned' to eat from the chromis in QT with them.
That said, reports on them actually 'schooling' are very mixed. Some have had luck, but most seem to find that the chromis start to kill each other, till you're left with 2 or 3 fish.
For size, the blue-green's stay, 'small'ish.' They do still reach at least 3" and the Blue-reef's grow even larger, but seem to not kill each other off. Though Blue-reef's are 3-4 times as much and at $4, definitely Blue-greens...