Combining young and younger...

[debid]

I put some full grown hens into my 4 week old flock and then some 3 weekers in a week later. They peck a bit but nothing scary and 2 weeks in all seems to be fine. We keep an eye on them but the bigger hens just ignore the smaller ones for the most part.

Please don't confuse your good fortune with "the way it is". There are folks who have had fatalities when introducing hens and chicks cold turkey or even chicks and older chicks or hens and other hens. Sure, it might work that smoothly again but probably won't. Why not? Because adding the hens rather than the chicks meant that you weren't putting chicks into those hens' established territory. That makes a BIG difference.

 

[nbenevento]

I wouldn't combine youngsters with adults. I put them in pens right next to each other and when big enough I combine them. I have found in my experience it takes about a day to settle things. Combining chicks was no problem at these ages.