If you are getting grown chickens and putting them in with your others, it's advisable to have a quarantine period. There are nasty things that chickens carry to other chickens and it may not be obvious right at first. I bought a 10 month old BR rooster last year and I quarantined him for a month before introducing him to my flock. He had lice and favus, due to his substandard living conditions, and I had to treat both of those, which were not apparent to me at first, so the quarantine was necessary. So many diseases are things your birds may eventually recover from, but they stay carriers for the rest of their lives, so caution is in order. Putting birds in with your flock with no quarantine can lead to the loss of the entire flock if they are bringing something nasty with them.
If you are getting chicks from a hatchery, they naturally must be brooded then need to be almost as large as the ones you have before you put them together, probably 3 months old at least.