At that age, the temperature is not a concern. They are usually fully feathered out at 4 to 5 weeks old and yours are well beyond that. With their down coats, those temperatures are certainly in the comfortable range. Your problem, if you have one, will be in integration. You may already know all this about integration but I'll assume you don't.
Integration is a difficult topic because so much depends on the personality of your chickens, especially the older ones, and your set-up. Part of the difficulty too is in what you consider a problem. A flock of chickens, much like a herd of cattle or a pack of dogs, are able to live together in a social group because they establish which animals have dominance over others. That way they know without a lot of disagreement which animal can enjoy the perks that come with being the dominant one. It enables them to live fairly peacefully as a social group. Unfortunately, establishing dominance is usually not very peaceful.
When you put them together, the older ones will establish their dominance in the pecking order by pecking. It is often not pretty to see. If no blood is drawn, it is usually not a big deal, but there can easily be problems. Sometimes you have an older hen that is a vicious brute. She will go out of her way to try to kill the younger ones. Often, if you have one of these, the others join in the fun. Sometimes they are fairly docile and just do enough to establish dominance. That depends on their personality.
I think your set-up is extremely important in how successful you are in integration. If the young ones cannot get away, the odds of something bad happening really goes up. I'll go through a few things I consider important. Maybe you can adapt some of them to help improve your odds of success.
If you can house them together but separate for a time, say about a week, that seems to help a bunch. Is it possible to keep the younger ones in a cage or fenced off area in the coop where they can see and be seen by the others, but the older ones cannot get to them? Putting treats on the ground where they eat next to each forces them to interact a bit. They may look like they are ignoring each other, but they are really not.
When you do mix them, the older ones will quite likely keep the younger ones away from the food and water. The young ones will go hungry and thirsty instead of challenging those mean old hens. If you can at all, set up different eating and drinking stations for a while, probably for a while. The young ones will correctly be afraid of the older ones for a long time. Most chickens are just naturally bullies. That's just the way they are. When I set up different eating areas, my older ones prefer to eat the food from the area set up for the younger ones, even if it is out of the same bag. You need to keep all eating and drinking stations well supplied during this phase so the young ones can get some food and water. You may need to be creative to put them in areas that the young ones can get to without having to challenge the older ones, depending on your set-up.
Give the young ones a chance to get away. The more space you can provide the better. Ideally you can let them free range so the young ones can stay away from the older ones, but many people don't have that possibility. If space is limited, put things in the coop and run that the young ones can hide behind but be careful not to provide areas that they can become trapped in. The older ones may be distracted by something new in the coop or run, so they spend less time harassing the young ones. It gives the young ones a chance to be out if sight of the older ones.
One lady on this forum stands guard with a garden hose when she integrates. When a hen attacks another, she squirts them. I have not tried it, but it sure sounds like it could distract the attacker.
If you notice one hen in particular being aggressive, isolate her for several days. This will knock her down in the pecking order so when she comes back, she is so busy working out her own pecking order issues that she may not bother the ones below her but instead concentrate on the ones that are now above her.
One of the worst times for disputes is when they go to the roost. If you can keep the younger ones where they sleep separate from the older ones at night but mingle during the day, that can make life a lot easier during integration. After a while you can move them into the same coop and on the roosts together, but the young ones will probably roost as far from the older ones as they can. Extra space on the roosts really helps.
Some people recommend to place the new ones on the roost at night after the others are asleep so that when they wake up together, they are one big happy family. I'm not a real big proponent of this philosophy. They still have to settle the pecking order. Sometimes that goes pretty easily and sometimes that is really vicious. I think one of the big advantages to this is that when things go smoothly, the issues are settled before the human is around to intervene or get upset. But if it does not go smoothly, then the young ones are trapped in a small area and cannot get away. That can lead to disaster. I think there is a way that this approach can be used, depending on your set-up. When they wake up, they are usually hungry. Sometimes they may hit the water first, but mine almost always hit the food first. If you are out there early enough to let them out of the coop and run before they wake up enough to go after the young ones, they may leave the young ones alone when they wake up and go eat first. The young ones will probably stay on the roosts until the older ones clear out. How successful this is depends on your set-up and the personality of your hens. You may have Attilla-Lolita the Hun who thinks it is more fun to attack the young ones than to go eat. The only way I would try this si to make sure I give the young ones as much chance as possible to get away when they do wake up. I have done this and it went OK.
The older your young ones are, the less chance of disaster. Many people wait until a minimum of 16 weeks. I don't. A broody hen will wean her chicks long before they are 16 weeks old. The other hens will peck at the young ones to keep them in their place and the young ones are often scared of the older hens, but they work it out.
I'm sure there is more, but this is long enough. What you do and how successful you are depends on the personality of your chickens and especially how much space you have. Hopefully you can pick something out of all this that helps you. Good luck!
