It is okay as long as the newcomers keep to themselves, but the problem is that some of our older birds seem to attack them when they get too close.
Which begs the question, what does your space look like? Do they have enough room so they can keep a distance. Size, in feet or meters, is important but also how it is laid out. Photos could be really helpful. Can you tell us more about when this occurs? It is a lot easier for me to give specific suggestions if I know what you are dealing with and what you have to work with.
In all my years doing this I've only had three cases where a chicken killed another so it is pretty rare. Two of those were where they killed a sibling, one set 15 weeks old the other 2 weeks old. The other occasion was different ages/levels of maturity. They generally kill by pecking at the head. They can cut a frog into bite-sized pieces with their beaks, they can drill into the head with those beaks.
Each chicken has its own personality, each flock has its own dynamics. These can vary a lot by individual and year by year. Some chickens can be pretty mellow about others while some can go out of their way to bully and attack any they perceive to be weaker. Less mature chickens are weaker in spirit than more mature chickens. Size does not matter, bantams often dominate full sized fowl.
It sounds like you are doing OK outside the coop, it is probably inside that is the issue. I'd suspect that this might have to do with bedtime but I'm guessing. With my flock, my juveniles do not sleep on the main roosts with the adults until the pullets start to lay. Until then they sleep away from the adults. My coop is big enough that they can do that. I personally don't care where mine sleep as long as it is predator proof and is not my nests. A lot of people do not have the flexibility in their facilities that I have. It makes my life easier. My general suggestion is to not force them to be together in tight spaces and give them as much flexibility as you can. For a lot of people that is not easy.
What sometimes helps is if you can identify a ringleader that is leading the attacking try isolating that chicken from the flock for several days to a week. This can knock that chicken out of its spot in the pecking order so when it comes back it is more worried about getting back in the pecking order than attacking others. This does not always work but it has a few times for me.
This one you probably don't want to hear. One of my goals is to hatch eggs and raise them for meat. It is not always easy to decide which ones to keep and which ones to eat. It makes that decision easier when one volunteers. Besides, I do not want to hatch eggs from that chicken. I do not want those aggressive bullying genetics in my flock. I much prefer a peaceful flock.
There are different tricks and methods for integration. I don't know which ones might be possible for you as I don't know what you are working with. People go through integration all of the time, usually successfully. I wish you good luck!