please quarentine the new bird for at least three weeks first as with any animal. besides disease, you could also spread parisites to your other birds.
i'm afraid to say that chickens are not more accepting. far from it in fact. they live in such tight knit communities that introducing a single new bird, particularly an older hen or a rooster, into an established flock can sometimes prove impossible. lucky for you, you have young birds and a much better chance of intigration. the best way to go about this i find is, after the QT (quarentine ), place the new chicken in a cage in the new chicken's coop and leave her there for a while so they can become accustomed to one another's presence in the coop. initially they may fight through the cage bars, but you have to just let them unless they are seriously hurting one another. break up these fights and there will only be worse ones later on, so it's better to establish the new pecking order through cage bars than out in the yard where there are no barriers or places to retreat. after a week or two, take them to a peice of neutral territory (this may have to be in your basement, neighbor's house, ect. and let the new bird meet first the lowest in the current peck order (you can tell the the order by watching your flock to see who is allowed to peck who and who is not), then the next lowest, next lowest, ect. let each establish thier individual peck orders with one another. after this is done, let them all together in the chicken coop/run. there might be more squabbling but it'll be considerably calmer than if you'd just thrown the new chicken in the coop and walked away. good luck with the new chicken!