You have three different types of agression to worry about when you integrate chickens.
If you have two or more reasonably mature roosters, they will sort out which is dominant. This normally (meaning almost always) involved fighting. Occasionally it is a fight to the death. Sometimes it involved a whole lot more chasing and running away than actual fighting. Often, they will settle the dominance issue and form a good partnership in taking care of the flock. There are a whole lot of different ways this partnership can work out, anywhere in between splitting the flock into two separate flocks or the two roosters becoming best of buddies and hanging out together. These partnerships don't always work out and there are occasionally refresher courses as to which one is dominant. My experience is that these are usually not fights to the death. Usually.
Then you have pure integration. Chickens can be territorial. If they see strange chickens they may try to run them off. You would think this would be the rooster's job, and occasionally it is, but usually it is the hens that do this. Those hens can be mean and brutal. Sometimes this can lead to chickens dying and sometimes it is not a problem at all. This is where housing them side by side for a while can be really beneficial. Sometimes it is not a problem at all even if they have not seen each other before. Sometimes you have a hen that will seek out to destroy any strange chicken even if they have seen them before. This is usually not a big problem, but it can be.
Lastly is the pecking order stuff. Each cow in a herd, each wolf in a pack, each chicken in a flock needs to know its rightful place in the pecking order so the social group can live and work together in peace. But the way they determine this is not peaceful at all. Sometimes they sort this out pretty quickly and without bloodshed. Sometimes animals die. Usually it is somewhere in between. Immature chickens are at the bottom of the pecking order and more mature birds will keep them there as long as they can through fear and intimidation. That's why you will see young birds in a flock keep as much distance between themselves and more mature birdes as they can. It is less painful that way.
With yours, you will find that the 12 week olds are at the bottom. They will stay away from the others if they can at all and have enough space. If space is tight, don't be too surprised to see them on the roosts while the others are on the ground or see them hiding behind or under things. When they mature enough to win their place in the pecking order, this will change, but not until they mature. This pecking order behavior is typical whether they have been raised by a broody with the flock and weaned or if they are introduced from a brooder. I've had three week olds that had been raised with the flock and weaned by their mother behave this way. They managed.
Your 17 week olds are tweeners. They will dominate the 12 week olds but will almost certainly be dominated by the oldest group. I can't tell you how old they have to be to win their place in the pecking order. I have very rarely seen a 15 week old win its place in the adult flock. Very rarely. I have seen chickens many weeks or months older not be mature enough to win a place.
When I integrate, I provide extra feeding and watering stations to reduce the areas of conflict. Let them stay as far away from each other as they can. The most important thing you can do, in my opinion, is provide them a chance to get away. If the loser in any of these fights can turn and run, you have a much better chance of this ending with chickens not hurt than if space is so tight they cannot get away. If you can't provide space, provide places for them to hide or separate from the other chickens. That might mean extra roosts or perches. It could be things for them to hide under or behind.
I can't tell you what the best way for you to approach it is. I don't know enough about your facilities. I just turn mine loose and let them free range together.
Good luck!!!
