I added a SLW cockerel to my flock of pullets - all large breed, 12-13 weeks old. I had him in a seperate area for a few days, all within visual (and pecking range through the fence) and then put him in the coop at night when it looked like things were settled out. I know more people like to wait for quarintine but I put him in after 4 days. A few standoffs and some pulled feathers but no injury, also I made sure to add a cabbage and a few new chicken toys to distract everyone. It's been over a month now and it's like he's been here from day 1. Those few days at first seemed to give them all time to figure it out, but I also do not free range my birds. Not sure if that would affect your situation.