I never hold to any rule being hard and fast.
With my Delaware flock that is separated by their run, I can put in young birds with little trouble. Frenchy hatched out four and from the time they hit the floor they were with her and all the other hens and the rooster. Then I put in three young birds a Del and Marans roo and a Marans hen. The younger roos were not a problem until I noticed the young Del roo snag Frenchy then I nearly killed him right then and there.
Now I had added 11 even younger chicks, Columbian rocks, Del and one EE. All was fine but when I noticed that Del roo hurting the chicks and Frenchy (biting and hanging on) I got rid of him and the Marans roo.
See the thing is the other Del roo chortles for them to come and eat, and he doesn't show the mean aggressiveness to the little ones.
NOW, in the Marans coop two little ones where born and both I found dead. In the EE coop a little on was born and that one too was found dead. So now I take any born and put them in a brooder since I've no where for mom and the babies at this time.
So what's my point? That not all chickens are created equal. Not all roos or hens are mean nor nice. Each situation and bird is different. You have to spend time with your chickens to learn their personalities. (if that is the right way to put it)
This is why you get varying opinions and ideas. So when someone says I do it this way or that, I just smile and say to myself, such is life.
For some good news, Buckwheat seems to be okay. Her stomach that was swollen like a beach ball has gone down. It was so swollen she looked like she was straddling a bowling ball. I'm so glad I didn't put her down. Yet all the books say whatever was wrong was fatal but her she is still alive. I carried her to the mailbox to get the mail. Poor girl hadn't been out in weeks. I figure if the bible says God knows every sparrow that falls, he cares about my chickens too, so I pray for them and I'll confess to even laying hands on my Buckwheat.
Have a nice day folks,
Rancher