Integrating new chicks with an older flock

I recently purchased 8 chicks; 3 reds, 3 americaunas, 1 barred rock & 1 buff. I've kept them separated, for the most part, from my older, year & a half-old flock of 3 RIRs, 2 buffs and 3 Barred rocks. I've tried to integrate them in with the older ones but the older ones are so mean and chase and peck the chicks even to the point of bleeding, one pretty severely. These chicks are now about 2 months old and are getting pretty big, but they still can't hold their own against the big meanies. What do I do?