I usually put chicks in their own small pen inside the coop at night so that the older chickens can see them, and get used to them, but cannot hurt them. If you don't have enough room in your coop to do this than try putting them inside of a cardboard box in the coop at night so the older birds dont feel like they are invading.

It sounds like they were fighting over their night time roosting areas. Chickens have their favorite places to sleep. When you introduced the new ones they might have been getting booted out of the established chickens spots.
You might want to put them into the coop once the other chickens have settled in for the night and are already asleep. This will give the new ones a chance to find open areas for them to sleep in.
Or, put in a new place for them to roost on, inside of the coop.
Good Luck!

Do you think my chicks are too young to join the rest? My older hens are pretty big (barred rocks) and a few assorted bantams and an Aracauna. I am planning on moving my younger chicks to the barn for the time being. I may try letting them all free range a bit together while I watch them. I don't have a big enough coop to section off a space, but I could try the box I suppose. I just feel so badly about the younger one's getting thumped on. I thought they were big enough.

I've got info on my chicken site here on the technique I use, it's a gradual 'Meet & Greet' type thing and works pretty well:

http://jackshenhouse.com/VSChickIntroducingNewBirdsToFlock.htm

There is naturally going to be some butt kicking of any newcomer, but you can help reduce it a bit. :)

Velvet ~:>

Velvet Sparrow, your website is great. You have some beautiful looking chickens there. I will try the gradual "meet and greet" and see how it goes. The chicken that got thumped on the worst is doing better and can go back with her group tomorrow.

Thanks,
Bonnie