1 week and 5 week old chicks - can I mix - help

I am new to having chickens. We got 4 chicks 4 weeks ago (they were a week old) and love them. We wanted to get a RIR when we purchased them but they were all spoken for. Today the store got more chicks and had RIR, in so my hubby got me one. She is only a week old and wow she is so much smaller than the other girls -- they have grown fast. My other girls are all hens (a Orpington, Australorp, Wyandotte and a Ameraucana). As we really wanted a RIR I was like yes yes yes, especially as I had read that hard to mix older hens with new ones. I just didn't realize how big my girls had gotten. My questions are...

1. Can I combine them now or do I need to keep them separate?
2. If I don't combine them when can I?

Please note that we live in town and my girls were in the garage until about a week 1/2 ago and I just moved them to the coop (we just finished it) and their run. They were supper excited when I put them in it. I did put them in a couple hours at a time each day until they went full time.

3. Should I do that with the RIR

Oh and the RIR is looking like she is going to be my escape artist.

Any advice or help is appreciated.