help! introducing new chicks this spring and need second coop!

I have three mature girls who are comfortable with their small coop, roomy run, and access to my big backyard. We have 5 babies coming the first week of May -- three regular size and two bantams. I am shopping for a second coop (either assembled or mostly assembled), and was planning on attaching the new coop to the side of the run opposite the existing coop and putting up a temporary screen in the middle of the run until they're ready to be together.

My big question: Can I get a coop that fits only the 5 newbies, or once they're all used to each other, will they all head together into one coop?

I can't spend a fortune on a big, new coop that could house all 8 of them.

Thank you for any and all help!