You have three different types of aggression to worry about. There are some interrelationships between the three, but there are differences.
You have the basic integration. That's where chickens recognize other chickens as members of the flock. If they see a chicken that is not a member of the flock in the flock's territory, they may attack them and try to run them off or even kill them. That's where housing them where they can see each other for a while is such a good idea. Each flock has its own dynamics. Some are open and welcoming of all strangers. Some have chickens in them that attack strangers. Often this is a hen but it certainly could be a rooster. A good rooster is supposed to protect his flock, but a rooster could very well welcome additional hens to his flock. Each flock is different.
The second type of aggression is pecking order. Each chicken needs to know its rank in the social order of the flock. Once the pecking order is established and the chickens understand chicken etiquette, the flock can live together pretty peacefully. But establishing that pecking order can sometimes be pretty violent.
It is perfectly acceptable for a chicken higher in the pecking order to peck a chicken lower in rank that violates the dominant one's personal space. That's how they maintain the social order. The dominant one pecks, the weaker runs away, and order is maintained. If the weaker does not run away, then it is a challenge that must be met. That's where space is so important. If the weaker cannot run away, it is seen as a challenge and it can get pretty nasty. Maturity has a lot to do with social rank. A more mature chicken will be higher in the pecking order than a less mature chicken. Once the younger chicken catches up in maturity, they may challenge the current pecking order to move up.
You will find that a younger group of chickens like yours will try to avoid the older chickens. They know they are socially inferior and that they are putting themselves in danger by entering the older chickens' personal space. This is another reason that having adequate space for them to hang out separately is so important. It is also a good reason to have different feeders and watering stations around, so they can eat and drink without challenging the older chickens. Having a separate sleeping area also helps. My flock is pretty laid back when it comes to integration and pecking order, but it does sometimes get vicious on the roosts when they are going to sleep.
Broody hens sometimes wean their chicks as young as 4 weeks. These chicks are already fully integrated with the flock but are at the bottom of the pecking order. Usually these 4 week old chicks mingle with the flock just fine, but they are at risk for pecking order issues. By the time they are weaned, provided they were raised with the flock, they pretty much understand how it works.
The third type of aggression is between roosters for flock dominance. As they mature and the hormones get to flowing, they decide who is boss. Sometimes that can get pretty violent, but often when they are raised together, they work it out without too much violence. Again, each chicken and each flock is different. There will probably be more challenges as yours mature, but usually these challenges are a quick face off, then a lot of chasing and running away. But it can get violent and roosters can get hurt. I have never had a rooster severely hurt in these challenges, but it does happen.
I know this does not tell you what to do, but maybe it gives you some ideas of what is going on and what to look for. Hope it helps some.
