Here is how I am integrating my two flocks....

Tonight I put out my newest four chickens and they are 8 weeks old. I have had them in a brooder in the garage since early November but their "annex" is now finished and I wanted the space in my garage back.

My DH built a separate square cube out of chicken wire for these guys for the next few weeks while I get them integrated with our older flock. They are now in the "visible but separate" stage, which I will keep them in for a week. This "annex" is cable tied to our large run and is about 4x4 with a hinged top lid. The larger run and the annex both have just wire between them so the old flock and the new one can see eachother at all times when they are out. Today they squawked at eachother and one hen in particular seemed very noisy and alarmed (will be watching her especially) but I fed them mealworms and kept them somewhat busy while they got used to eachother on separate sides of the fence.

In a week, after a good time period of staring at eachother, I will start to actually "mix it up":
DAY ONE: After deciding who might be the easiest hen to release with these young ones, I will let the new flock free range with her and see if she seems fine. Then, four hours later, I will let out another of the most docile ones (I am sure you can already pin point which ones you would put out--my easiest one has always been the easiest!). Now there will be two older and four newer. The other four older hens stay in the run during this time. The last part of the afternoon, I will swap both groups entirely--the ones out will go in and vice versa, with the young ones back in their own fenced in annex.
DAY TWO: Repeat above but with shorter time between letting out the older ones. Add one more hen to the mix. Today there are three older and four newer out. In the later afternoon, put all newbies back in their annex and open up older hens to graze the rest of the day together.
DAY THREE: Let out all newbies for an hour, then let out all the olders. Stay outside and watch for at least a few hours. Good time to do some Winter raised-bed garden prep! If there's an altercation, I will be there to do some separating. Anyone too aggressive goes back in the run (or in the annex!).
DAY FOUR: Now we start working on getting the newbies acclimated to the new run and coop. I will let the olders out but put the newbies in the big coop/run for a few hours. They will get a chance to have this area to themselves to explore and go inside the coop, etc. They will stay there most of the day. Most likely the olders will at some point gather around the outside of the run and squawk, "Hey what are they doing in our house?".
DAY FIVE: After a few hours of everyone out free ranging, I will start again with the Day One procedure, putting only the oldest most docile hen back in the coop area and add the newbies. Last time I did this, this is where things got a bit touchier. The first sweet hen had no problems with being in the run with the newbies, but by hen number three, there was some pecking. I prepped the run beforehand with three feeding stations on all corners of the run so food was available and at opposing corners. I think this helped a lot!! I also threw in mealworms to distract them. Keep these three feeding stations for at least 5 days. The less competition for food, the less squabbling I think.
DAY SIX: Same as above but with two older hens. I also give them plenty of time all together outside free ranging which helps too.
DAY SEVEN-TEN: Same as above but with three older hens. I will also mix it up even more, confusing them by putting two older and two newer together, and mixing up the combinations. Pretty soon they just don't remember who was there "before" and who was new. There will always be a pecking order but this will hopefully help them all to find their place with minimal fighting.

The annex might be a useful "time out chair" for any hen who just can't get along....last time I used a dog crate inside the run when one hen in particular just wouldn't stop pestering another (we call her Miss Meany for a reason!). This schedule seems somewhat onerous and I know some of you just pop the newbies in at night when they won't notice. But, I have had success with this and don't mind all the swapping around. In the end, I just want as harmonious a flock as I can.

BTW: the "annex" was made to fit exactly over one of our raised beds in the veggie garden. The intent is to serve as protection from birds and the hens for the veggies that will grow there. With the open bottom and the hinged lid, I think it will be great for a dual purpose.