We had the same experience as you a couple of years ago. We had 4 Zwartbles who all came together and were by then 9 years old. We were asked to take on an almost blind, crooked headed Beulah Speckled Face. We took one look at her and couldn't say no! But we worried that she would be terrorised by our big 4 so we thought it would be better if she came with a friend. So 2 young Beulah Speckled Faces arrived. We quarantined them for a week, and then moved them so they could touch through a fence for another week. Then we let them out.
Well if I hadn't been worried about heart attacks and broken legs it would have been funny! The two young white ones went bananas chasing about the field and our poor old black ones didn't know what had hit them. The new ones ran up to the old ones, who ran away. So the white ones chased, and so it went on. Maybe they didn't know they were sheep as well, I don't know. But we had to separate them again and then introduce them in a supervised way, and in a small area so there could be no running about. It took a while!
However, a couple of months ago we bought 4 new Zwartbles. We quarantined again, for 2 weeks (a bit extreme maybe), and then put them all in together. And nothing happened. No running, no chasing, no head butting.
So, my advice, incase there's going to be trouble put them together in a small area where you can intervene to break anything up. Of course, there's probably be a school of thought that says put them all together and let them sort it out for themselves. I just couldn't bear to watch!
Good luck!