Let me give you a link to Buff's article. We may not always do things exactly alike but I think her article is full of great information. I think it is well worth your time to read it.
Buffs Integration
https://www.backyardchickens.com/web/viewblog.php?id=2593-adding-to-your-flock
Some ideas that might make the integration easier.
House them side by side for several days, where they can see each other but not get at each other.
Provide separate feeding and drinking places for a while. The older chickens will probably keep the younger ones away from the food and water as part of the intimidation process in setting up the pecking order.
Give them as much room as you can. The older chickens will be higher in the pecking order and will almost certainly jealously enforce that position. What that usually means is that the older chiken will intimidate the younger and the younger will run away. If the younger does not have room to run, the older thinks the younger is challenging them for pecking order domninance and will very vigorously enforce their position.
For the first week or so, make sure the pop door is opened early. Don't leave them locked in a coop where the younger ones cannot get away. What I normally see is the younger ones stay up on the roosts and the older get on the floor of the coop, but sometimes an older one will stay up top and go after the younger ones.
Make sure you have plenty of roost space. We all have different circummstances, but I find mine are most aggressive when they are settling down on the roosts. The younger ones need sufficient room to get away from the older ones or they might start roosting in nest boxes or something like that.
Buff recommends waiting until they are 16 weeks old to start. I usually have mine fully integrated by the time they are 12 weeks old, but I start when they are 8 weeks and work at it. I free range mine so they have a lot of space and I am around all day to work on it. Some people have them fully integrated at a much younger age. Differrent things worek for different ones of us.
Good luck! Sometimes this process goes so well that you wonder what all the fuss is about and sometimes chickens die during this process. I really think having sufficient space is a big part of this.