Also introduce them slowly. Keep letting them see each other, maybe have some in wire cage or something and others outside and let them look at each other and stuff. Maybe even for short periods of time let them play together, but then pen them back up separately. When they are big enough and they all seem to be getting along, then they can go out!! They will still chase some and establish pecking order.
Your chickens are soooooo cute!!
When your chicks are near the same size as your other chickens you can start introducing them. Around 16 weeks they should be getting up to size but some chicks take a bit longer depending on the breed - some stay on the smallish side. Before that they can't defend themselves and fight back when the new pecking order is established.
My 6 week old jersey giants and my mystery chick are with the adults now. At 4 weeks old they were in a large parrot cage in the coop where they could see everything. At 5 weeks I put them in a brooder cage on the floor. They could see the adults, and the adults could see them. After a few days I left the cage door open during the day. They moved in and out and mingled. A couple of hens each gave one peck, but nothing significant. The rooster actually seems to protect the babies. Now The cage has been removed and repaced with a small perch under a heat lamp. When they get cold or tired they rest there. However, keep in mind that I have no mean chickens, and my rooster is exceptional.
The 6 week olds are running around the coop with the adults. The 5 week old pullets are in the brooder cage on the coop floor. Five 4 week old giants are in the parrot cage. Five 4 week old giants are in the garage. And the silkies are in a brooder cage on the coop floor. I changed the adult's feed over to Purina Flock Raiser so that everybody could eat the same feed, and the babies would get adequate protein and nutrition. I just leave oyster shell out for the hens.