I'd separate the 'meat' birds pronto, that will provide immediate relief for the remaining flock.
I usually slaughter cockerels at 13-16 weeks, right before(or immediately after) they start causing trouble and still tender enough for the grill.
9' x 5' coop is tight space for 18 birds that get along........very tight for integration.
You said "one of the new roos"....maybe get rid of the males now(how many?) to reduce the population?
A "few days" of side by side is not really enough time.
Do you free range at all?
aarts integration...