Integration is a tough one. It can either go well at about any age or it can be a disaster. How it goes depends on how much room you have and the personalities of your chickens. I can give you some suggestions to improve your odds of it going better, but please realize there are no guarantees.
Start letting them mix for short periods of time under your supervision, then extend that time period as you get comfortable with their actions. Some pecking and bullying is normal. That is how they establish the pecking order. Allow them to do a little, but be prepared to break it up. One member of this forum uses a hose and sprays the aggressor to tell them that is not allowed.
Provide extra food and water stations. The big ones will keep the young ones from eating and drinking in THEIR feed and water stations. Put a few and make them as far apart as you can get them. This will cut down on conflict and allow your young ones to eat and drink. Watch this and make sure they are eating and drinking.
Your young ones will be scared of the older ones and will hang together in a group as far from the older ones as they can. If the old ones are in the run, the young ones will be in the coop. Try to give them a safe place to hang away from the older ones. If you can set it up so they can get into the pen they are in now, so much the better. Just give them access to the big ones area so they can venture in if they want to. It would be great if the opening were sized so the yong ones can get through but not the older ones, but that is rough.
If you can set it up where the young ones are sleeping separately from the older ones, good. If you cannot, make sure you get up with the chickens and open up the coop early so the young ones are not trapped in with the older ones when they wake up. Give them as much room as you can so they can get away from trouble.
I kept my brooder in the coop. They could always see each other. At 11 weeks I let them free range together. Lots and lots of room. By the time the young ones were 12 weeks old, they were sleeping in the same coop on the same roosts. The young ones were crammed together on the far end of the rosots as far from the older ones as they could get, but they were sleeping on the same roosts. And if I was a little late letting them out in the morning, no one was hurt. The young ones stayed on the roosts while the older ones went to the floor of the coop.
Yours may be a little young, but others have done it even younger. I think a whole lot of whether you are successful or not depends on how much room you have. The personalities of your chickens makes a big difference too. You may have a bully that will go out of her way to harm the young ones. Some roosters will take the young ones into the flock and protect them by breakig up the fights when the hens go after the young ones, some may attack them, and many will just ignore them. I personally worry more about the hens than my rooster, but some roosters can be real dangerous in these situations, especially if there are roosters in the young ones that are starting to feel their hormones. There are no guarantees in any age or sex mix. I'd be uncomfortable at 7 weeks but some people make it work. Good luck!
