I would do it soon. It will be quite a bit easier with them being so young and not that far apart in age. This is my integration procedure: After quarantine, (I usually buy adult or started birds from other people. If you do this in the future, you'll want to quarantine as well to prevent the spread of any diseases the new birds may have to your flock.) I let all the birds free range together for about a week, depending on how things go. When they're not on free range, the new birds are in a pen alongside the fence to my coop so they are within constant sight of each other. After ranging together and I think all is ready, I slip the new birds into the coop at night. And I mean at night. I usually go out between ten and eleven o'clock at night. The oldies wake up to find the newbies with them and there is some pecking, but usually not too serious. In your situation, nine week old chicks have a pecking order, but they can't really do too much harm. I'd let all of them out together, and after a few days, you'll probably be ready to completely integrate. Good luck, and I'd love to know how it goes. Don't hesitate to ask anymore questions you may have!