I am constantly adding new chicks & adults to my coops. The key is to add them in groups of 5 or more so they can flock together for protection if they are younger. Adults are more of an issue than chicks, especially roos. If you get more than 1 adult roo in the coop, they will likely fight to establish dominance. If the roos have been raised together from chicks then it is much less of an issue. When chicks establish the pecking order the squabbles are short & non-violent. They do a bit of chest-bumping & pecking to show who is in charge & then all is settled again. Adults may actually injure each other if not separated when squabbles arise. Also, adding new birds to the coop at night helps. Chickens don't have great memories so when they wake up together they tend to think they have always been there. I try to check on new birds frequently the first few days so I can head off problems quickly if they do arise.My chicks are thriving. I will put them out into the coop at 4 weeks. They are my first batch of Day olds-- I have some older ones I got at ages 1 and 3 weeks- that are already outside-- they are fine as well- my main concern now is mixing the 2 groups ( I want the chicks out of the house! But I only have one Coop...) I wanted a local Hatchery and I am in Northern CA so went with one not far from here called California Hatchery-- it was free shipping for 15 so I got 15...All 15 arrived well and remain well, they came 26 hours after hatch time so I do believe that was a factor....