If you are never going to let them establish a pecking order, then you can never integrate them. Establishing a pecking order involves pecking.
There are a couple of different things that happen when you integrate chickens, integration and pecking order. If roosters are involved, there can be a third but I think yours just involves pullets. One way to explain the difference may be that when a broody raises chicks with a flock, they are fully integrated and part of the flock when she weans them, but they have to establish their own position in the pecking order when they mature enough to do so.
Sometimes integration can go so smoothly that you wonder what all the fuss was about and sometimes chickens wind up dead. Each individual chicken has its own personality and each flock has its own dynamics.
On integration. That is where the chickens do not initially recognize each other as belonging to the same flock. They can fight over this, sometimes very seriously. They will protect food and water resources from the other flock, as well as their territory. One way to help overcome this problem is to house them next to each other for a while so they can get used to each other, usually involving a fence between them. Throwing scratch or some treat on the ground where they eat next to each other can help them get used to each other. Sometimes a flock has a chicken, usually a hen but it can be a rooster protecting his flock, that actively seeks out the weaker intruders to do serious harm to them. Most flocks don't have these chickens, but some do.
The other issue is pecking order. Each chicken in a flock has to know its social position so that the flock can live together in peace. Unfortunately establishing this position often involves violence and pecking. The more dominant chicken has certain rights within the flock and establishes and defends those rights by pecking when a less dominant chicken invades her personal space if she feels she needs to protect her rights. Most of the time, after the pecking order is established, they can get right next to each other and nothing happens, but every now and then, one will peck another to maintain her social position. If the chicken being pecked moves away, she has submitted and all is peaceful in the flock again. If she does not submit, a fight for social position can happen.
Size does not make much difference in establishing pecking order in mature chickens. It is more the spirit in the individual than the size. When immature chickens are involved, maturity plays a big part. The more mature chicken will dominate the immature. Your 12 week olds are going to dominate the others because of maturity more than size until they all grow up. As the younger ones mature, they will establish their own position in the pecking order. Occasionally this may involve fights, but usually it involves some pecking and a lot of moving away. Almost always these pecking order pecks are not meant to kill. They are meant to discipline. It is a vicious peck and a chicken can be injured, especially if there is a big difference in age or size, but injury from these pecks are really very rare. It is when the discipline turns to a challenge and a real fight that it becomes dangerous.
Some dangers involved in pecking order are when two chickens can't decide which one should submit, so they fight it out. That should not be a big issue with yours since the older will dominate. Sometimes, you have that chicken that does not know when it has won and keeps pursuing the submitting chicken. These chickens are a real danger to the tranquility of the flock. Deal with them as you will. I have not seen many of them, but they occasionally exist. To me, the most likely danger is if you do not have enough space for the submitting chicken to get away, the winner does not realize she won. She sees the other chicken that is trying to get away as hanging around to challenge her. Sometimes a chicken can get trapped against a fence or on a corner and cannot get away. To me, giving them as much space as possible when you integrate is the most important thing you can do to help make integration as peaceful as possible.
What you can expect to happen is that the young ones will stay as far from the older as they can. If they free range, they will find their own hangouts. If they are in a coop and run, you can expect the young to be in the coop when the older are in the run and in the run when the older are in the coop. The young may spend a lot of time on the roosts up out of the way. They will avoid eating and drinking if the others are around.
One other thing you can do is to give them separate feeding and watering stations to avoid the conflict to start with. Try to give them extra roost space. Mine can be pretty vicious on the roosts in protecting their personal space. Don't leave that pop door closed any later in the morning than you have to after they wake up so they are not packed together in a small space. I find it beneficial for them to sleep in separate quarters for a while when they first start the integration.
I know this is long and rambling, but i don't know how to explain this without a lot of typing. But I get the opinion that you think any pecking is bad. Some pecking is going to occur in an integration whether you are around to see it or not. They have to establish that pecking order. Pecking order pecking is not that dangerous unless it turns to fighting or the weaker chicken cannot get away. It is when you have those mean, vicious, seek out to destroy hens in your flock that integration and establishing pecking order can be really dangerous.
Good luck! I hope this helps you some.
