I have 18 ISA Browns at 16-18 weeks of age. They have been with 10 older layers and a rooster that have mostly accepted them though the older hens can be testy in the hen house running them out once in a while but they do roost together without incident. They all free range during the day but...
Put 12 10-week old ISAs and EEs in their new home with 10 old hens and a their Rooster. Been a bit of a standoff but apparently the young ones know the rooster is their best ally right now. They pretty much mob him at bedtime. He doesn't seem to mind. Another reason I like a rooster around.
Went out to close up and our resident 'possum was chewing on a bone near the coop. Usually sticks to the sunflower seeds out front but 24" of snow calls for desperate action I guess. The chickens ignore him mostly.
No we did not kill him. He's been hanging around for years.