I adopted 4 adult hens when my 4 RIR were 6wks old and i kept them separate for 2 wks (not the full month that is recommended which I would have done if I could have, long story, anyway,) Then I put my younger ones in the coop but sectioned it off so everyone could see everyone but no one could hurt anyone and had them like that for 2 wks. Then at breakfast time when the 2wks was up I let everyone out to range together. The new hens were way more interested the foraging then the 4 RIRs and at roost time that night everyone when in the coop and found their own roost perch. I have had no problems. They sorted their pecking order out but no injuries at all. Then after a wk of peace in the coop I did it all again with 6 4-5wk old chicks. 
Everyone is living together now (all 14), took time and patience. I don't know if I got lucky or what but they have all gotten along. Everyone seems to stick with their own groups. The 4 RIRs, the 4 older hens and my newer 6 each stay with each other. but now that all the younger ones are about the same size they have been mingling together. Treats and regular dust baths helps with the unity and I have a mix breed bantam type hen that is the queen whom all the others sort of follow her lead. So dotes on the younger ones and the 3 other older hens respect that. 
It's been a great learning experience and mostly I think each person has to do the best they can in the research department. This forum has been great!