I have two green chromis (green chromii?) in my 55 gal. tank. I'm
considering adding a couple more. Should I expect aggression, or will
the current chromii accept their new tankmates with open fins, or?

Thanks in advance.

Green chromis are aggressive towards each other. We cannot keep them in a
big enough tank to keep the aggression spread out as it would be in the
ocean. I had 8 fish in my four foot set up. When young they interact
spectacularly, but as they mature, the weakest is systematically picked off
until you are left with only one. It may take a year or so for this to fully
happen and s lot of keepers will usually just replace the once that has
died. This in itself will bring out the aggression in the group and the
newcomer will be hounded to submission. Personally I love the fish. How many
more are you thinking of adding? At least three should go in and that way
the aggression should be equally dished out to the newcomers.

Regards

Mark
www.marksfish.me.uk