You're facing two problems, at least. One is that the "older" hens are still borderline juveniles. They will be in competition with the two-month olds. The youngsters are naturally afraid and will concede to the older ones in all things, refusing to stand up for themselves at this time. It's how chicken society is set up.
I said that you need to level the playing field. That means you need to provide advantages for the chicks. This means rigging up what I refer to as a "panic room". Since the chicks are already of size, it won't be functional but for two or three weeks, but it will give the chicks time to gain self confidence.
You will cut 4 1/2 inch by 7 inch holes in this temporary safe pen so the chicks can come and go but the older hens can't. Food and water are placed inside for the chicks so they get plenty to eat and drink without being chased and hassled.
By the time the chicks are ten weeks old they won't be able to fit through these portals, but they should be able to hold their own by then.
As for sleeping, go ahead and put the chicks in the coop with the older hens, but wait until the hens have settled in. The chickens won't be much interested in tormenting each other at night as they will be much more inclined to sleep. It should also be too dark to see. If not, you need to do something about that.
Place the chicks on a perch as far from the older girls as possible. You will also need to teach the chicks to go into the coop at night. They won't just automatically know to do that. Putting them through the pop hole will teach them, and they should be doing it on their own in just a few nights.
Yes, you will need to referee for a week or so at bedtime, but that's part of the duties of a chick parent.