I am a fan of stick them in at night and let them wake up together if they are full grown. I also never add just one chicken. I try to introduce balanced groups so that neither wants to pick fights with the other. Me being a control freak, I am up at the break of day on those occasions to see how it goes, but every time I have done it, it has worked beautifully. Mild pecking, but sorted quickly. With adding in young birds I would not do this until they are fully grown. My coop has an interior door that connects or separates depending. I place my younger birds on one side until they are full grown. they play in the run during the day while my mature chickens free range. They can see each other, hear each other, ect. Then when fully grown, I will go in at night and intermingle all the chickens, and set my alarm for in the morning to see referee if needed.