I agree with this for anthias that the hierarchy is mainly gender based. Male anthias is the most aggressive and the dominant. There is also female to female aggression but that is low as long as there is a male. And normaly you should only have one male in the mix. So one male with 2 females divide the aggression to 2.
But in chromis hierarchy is mainly based on size (males as slightly bigger but a large female can still be bigger than a small male), so the bigger fish try to dominate the smaller ones. Unlike anthias where you can have a single male, you cannot have 3-4 chromis where the is one big chromis. There will always be certain size difference between them. So like there will be biggest, bigger, big and not big (aka punch bag)
. So in chromis always had better track record with 2 chromis over 3 (or for that matter even till total number us at least 8).
When I had 3, a linear hierarchy of "1st-2nd-3rd" forms and the least dominant 3rd fish gets beaten up by 1st and the 2nd. In addition to this, since the 2nd fish shows domination behavior and colors towards the 3rd fish, these kinds of dominant behavior provokes the 1st fish to beat the 2nd fish.With just two, you end up with one dominant and one submissive fish. Since the submissive fish doesn't have a 3rd fish to exert its dominance, it doesn't show dominant behavior or colors, so the dominant fish doesn't constantly try to pressure it.
When you add more chromis, this situation gets even worse because again a linear hierarchy is established but this time there are more partially dominant fish in the middle. To make things worse, some fish try to move up in hierarchy by fighting and if that happens, the whole group try to establish a new hierarchy again. This cause constant squabbling.
However, when there is more than 10, they can no longer form a strictly linear hierarchy and there is too many fish divide up the aggression and provide diistaraction. It gets more unlikely that two fish that dont get along always founds themselves close to one and other.