when introducing new birds to a flock there should be a period of time where there is no direct contact between them, like a full chicknwire enclosure for the smallers inside the larger chicken's home.
Chicks dont fully feather till 5-6 weeks and cant make it with the olders outside in extreem weather and should be coddled till then. they are not even close to big enough to attempt to find their place in the pecking order, which will cause lots of disturbance now and later when they finally get big enough to do so.
also could your one mean chick (at 3-4 months) be a cockerel? check out if this bird is top bird, if it is and is overly agressive you may want to think of culling it. some hig agressive to other birds birds become agressive to people as well. And that is not Fun! I had a wing clipped rooster that would jump a 6 foot fence just to attack me, even when I was bringing food or treats.