Welcome to BYC!
What you're seeing is normal...they're trying to work out the pecking order of the flock. Even if the birds are the same age/size, introducing birds new to each other will cause the antagonism.
With new arrivals, it's important to first hold them in isolation for 3-4 weeks so you can be sure you aren't introducing an illness or pest into your existing flock. Then this has worked very well for us...before mixing them all together, have a period of "look/don't touch" for a few weeks. That way everyone can get used to each other and your existing flock won't be so likely to view the newer girls as strangers. There will still be some skirmishes happen but as long as no blood is shed or a serious injury, they'll work it out.
Good luck with your new, bigger flock!
and Welcome To BYC! X2, it is best to have a long period of the two flocks being able to see each other but no touching, through wire seem to work best, ie dividing the coop into two sections or keeping the new/younger ones in a cage inside the coup for a couple of weeks to a month at least. The chickens will get to know each other and sort of work out a pecking order before actually coming in contact with each other. After a week or two, letting them free range together is a good idea and should help... It will take a couple of weeks to get the pecking order sorted out. There is a nice article in the Learning Center on integrating flocks you might like to check out, the part about actually combining them is after the quarantine section https://www.backyardchickens.com/a/adding-to-your-flock and a nice article using the wire method https://www.backyardchickens.com/a/introducing-new-chickens-using-the-see-but-don-t-touch-method
Greeter
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Spirit
