To me it’s almost a coin flip. Why you want a rooster may play into the decision.
You don’t have any way to know how either will really act as dominant rooster. The young one is just too young for the hormones to kick in as Pyxis said. But the older one is not currently in a dominant situation. The other two are obviously dominant so he does not have the responsibilities of a dominant rooster or the freedom to act out if he wants to. If he is put in with your pullets he will become dominant. You don’t know how that might change him in how he protects the flock or treats the pullets. You just can’t tell in the situation he is in whether he will be human aggressive or not.
I’s hard to know how the older one will interact with your 12 week old pullets. He will immediately be the dominant one but the pullets are still chicks. A good dominant rooster takes care of all members of his flock, so I don’t think he is likely to try to hurt them, but he will be dominant and may feel a need to show he is dominant if some of the pullets don’t accept him. He may not want to hang with a bunch of pre-teens but he probably will. They may not want to hang with him and try to keep their distance. They will probably integrate OK but there will be some interesting dynamics going on.
Another thing is that your pullets are not yet sexually mature, but aren’t far away. A good mature rooster will wait until they mature a lot more before he tries to become sexually active, but there is a kicker in this. The mating ritual is not just sexual, it is also about dominance. The one on bottom is accepting the dominance of the one on top, whether willingly or by force. It’s quite possible when they are introduced that he will mate with some just to establish his dominance. This should not hurt the pullets but it is upsetting for some people to watch and the size difference does add some risk. One way to get around his risk is to quarantine him for a month or six weeks. I don’t think this is likely to show any diseases he might have as long as the flock he is coming from is healthy and stable (no new chickens introduced for the past month) but it might. The big advantage would be that the pullets would be a lot more sexually mature and larger when they are introduced.
The older one will be much more ready to start as flock protector, looking out for danger and in general taking care of his flock. The younger cockerel will need months of maturity before he is ready to take on this role.
The older one is in the prime of his life, but in two to three years he will start to get old. His energy and sexual drive may begin to slow down. You will probably need to replace him sooner than the 4 week old if fertile eggs is one of your goals. You’ll probably need to replace your hens too about that time if eggs is your goal. They tend to start slowing down in egg production at three to four years old. Raising your own replacements may be one of your goals so this may not matter.
With your pullets 10 weeks old and the young one 4 weeks, it’s a bit risky to try to integrate them now. It can be done, especially if they have a lot of space, but watch and see how the pullets treat him. I think the time that might be hardest on you might be when the cockerel reaches puberty, say 12 to 16 weeks old. Onset of puberty for cockerels can vary but if he is the only one it is usually earlier rather than later. His hormones kick in and he wants to become sexually active. To do that he has to dominate the pullets, physically but also by maturity and personality. Some hens will squat for about anything wearing spurs but many older more mature hens expect a rooster to perform his duties of finding them food, dancing for them, protecting the flock, and keeping peace in the flock before they grant him any favors. That’s often referred to on this forum as the older hens schooling a young rooster, teaching him his manners.
The pullets will be older and probably about the same size when he hits puberty. They may not be willing to accept his dominance without a fight. There is a tendency for older hens to be kind of brutal to an immature rooster getting above his station. Again, they will work through this phase, especially if they have enough room, but it can be hard for some people to watch. If space is tight it can get rougher.
I don’t know that there is a big advantage one way or another for you. The dynamics of the integration are certainly going to be different with each choice, but at the end of the day they will work it out. In either case you might want to have an isolation pen ready in case you need to give the pullets or the cockerel more time to mature. In either case it is also possible that this will go so smoothly that you think I’m crazy to even mention these things.
