help with introducing new hens

Hi everyone, I am in the process of trying to introduce 4 x 12 week old hens to an existing flock on 1 x 12 week old hen and 2 roosters. I have the intention of doing away with one of the roos but he's not very big as of yet. They have all been sharing a coop at night for 2 weeks now with no fighting and everyone getting food - (the coop is rather large). The 'old flock' free range but the newbies stay in (or very close to) the coop during the day. My issue is that they show no signs of mixing, if they stay away from each other all is relatively ok (no blood drawn) but if the newbies inadvertantly go near the others they get pecked or chased. As a result they seem reluctant to leave the coop at all and seem rather stressed if the newbies are around. Do I leave well alone and hope for the best? I did have the passing thought that if they were going to act as 2 flocks anyway maybe I should try to get one of the roos to switch sides and have 2 tiny flocks - but that really breaks the 12 to 1 rule! Does anyone have any suggestions?