Hey there,
I've heard of doing this a couple of different ways...
First, its usually best to wait until the birds are closer to the same size before you integrate them...I usually wait til my youngsters are about 3 months old, because by then they are a good size to let run around with the older birds.
Second, try integrating them at night, when you lock them up for the night....They're usually calmer then and more focused on finding a resting place for the night and less likely to pick on any newbies....Then when they wake up together in the morning they're more fascinated with the new ones than likely to get hostile with new 'invaders.'
Hope this helps.
Take Care,
Bill.