I thihnk if you add them at night, and if your poultry set up includes a place where little ones can run to to avoid confrontations with bigger ones (such as a nest box), then you should be okay. There may be tussling and a peck here or there, but it will settle down, and should do so without much trouble.
Thanks for the advice (and support) Uniontown! I haven't "combined" flocks before but...chicken math! Fortunately there is room for more in the big coop and the 8 smaller pullets went into the small coop!
I'd be really careful, that's a significant size difference. Have they had any time to get used to each other with some sort of barrier between them first? You'll also want something safer than a nest box for them to retreat to, and something they are comfortable in, as the larger birds can easily enter a nest box and then they are just cornered.
I tried integrating at that approx age difference, the younger ones had grown out in a fenced off portion of the run so they saw each other on a daily basis, and I'd even let them have a few supervised "visits" with the bigger birds. Then I tried the night time move. Next morning I went out early to check on them and one of the chicks had basically been scalped from the back of his head to between his shoulder blades. I had left a "safe access" back into their area that only the little chicks could enter, and that was where I found them, but that one chick obviously didn't make it there in time. I waited another month to let him heal and try again, with a little more success that time.
You should really wait until the babies are the same size or close to the same size as the other birds. 8 weeks is not all that big, wait until they are 14 weeks to be safe. If you add them sooner they have a good change of being hurt by the others.
Well I added them last night...this morning I am happy to report all is well. The older girls are out in the run like usual and the younger ones are hanging out in the coop. When I checked on them (again) the older girls came into the coop to see what I was doing and paid no attention to the young ones! I work from home so I can easily check on them frequently and the coop is close enough that if a tussle starts I will hear it!