I have done this several times, with different ages:
 
I have added adult chickens. I never have added just one. And I don't quarantine, to me healthy looks healthy. However, I don't buy adult birds from auctions or swap meets, but rather from other people who have chickens like me in similar set ups. It is a risk, but I don't have a huge number of birds or a valuable flock. I have several hideouts in the run. I have only had one bird be a bit cantankerous, but I let them sort it out, and eventually they did.
 
I have for years, let a broody hen raise chicks in the coop. With my first broody hen, I tried separating her, she went back to the nest she liked. I decided she knew more about it than I did, and gave her chicks. Since then, I let them do it their way. Sometimes I get chicks, sometimes I don't. The broody hen introduces them to the flock, and protects them from the flock until they get used to them.
 
Many people with the best of intentions, separate the broody hen, and the flock forgets her. Then they wait until the chicks are 5 to six weeks old. The hormones for broodiness are falling or are gone all together. At that time they try and reintroduce the whole works into the flock. The hen is a single adult bird being introduced to an established flock. She is in a desperate situation of establishing her pecking order, has hormonally forgotten her chicks, and they too are at the mercy of the flock. Put that broody hen with the flock when her hormones are high, and the chicks stick close to her and you will have success.
 
I do think that there is a very big difference, in a static flock. One that has not had change their whole lives. Flock mates raised together for three years for example are going to be very territorial. Flocks that routinely have had chicks in them, are much more tolerant of chicks, I think.
 
I successfully introduced brooder chicks to my flock just recently. They were less than 3 weeks old. I did not pen them up, but I did give them a safe place to retreat. Much like a broody hen, if you think about it, a successful broody hen is a place where a chick can go for a warm up and be protected from bigger birds. The chick needs to know where this is. I used lattice panels. My chicks could go through them like water. They were not trapped on one side, big hens on the other. The chicks can venture out on their terms, scoot back into protection when threatened. These chicks quickly learn chicken manners. It took less than a week, and there was no signs of strife.
 
Mrs K