Slowly. This is going to take time because of the age difference. Put them side by side so they can see each other for awhile. Later you can make a common area for all, but have some safety place where the babies can go and be safe. They will all have to sort out their pecking order eventually. I'm afraid that the older chicks might hurt the younger ones. You can try putting them together and see what happens. Whenever I get new chicks I keep the separate from the rest of the flock for at least a month. I then put them in adjacent runs where they can see each other. When I do finally put them together I let them out into the yard to free range and spread plenty of treats around so that the birds are more interested in the treat than killing each other. Some integrations go more smoothly than others. I had my birds in different coops. I switched the chicken around and put them in each others coop. I put the pullets in the hen's house and the hens in the pullets coop. Each coop has nest boxes. I kept them in separate runs also so they had to use the coop I assigned to them. I left them this way for a week. After a week I opened the runs up and let them intermingle between coops and for the next week let the girls choose the coop they wanted to roost in. The following week I closed the pop door on the pullets coop so that no one could use it and so they all had to roost in the hen's house. Well it worked with relatively little squabbling. Mission accomplished. They all get along fine now... This is how I do it. There are pictures on my BYC Page.