Get the agressive one away and put her in a dog crate or seperate area for a few days, let the new ones calm down and find their way around the coop. When you put the agressive one back in, she'll be less the 'top hen'... but will still need watching.
Let them try some more, if need be repeat the process. SOME pecking is inevitable...
Ahhh, it's difficult, (nearly impossible) to safely combine them if the new ones aren't very close to the same size as the older ones. At least as close as possible, and certainly fully feathered without any 'dib' fluff left.
As has been said, pull out the meanest older girls and isolate them for a few days so the youngsters can get used to things. They'll still be at a disadvantage, but it'll help.
LOL, ok you got me there, just so long as they are acting more like chickens and less like dibs then I guess. Clucking not peeping
and fairly good sized. I can't really say for sure... You've already started integrating them, so go on from there I guess!
If pulling out the most agressive full grown hen (or two) doesn't help, take the youngsters back out for a bit is all I can say.