Can I keep all happy?
Maybe. maybe not. The only way to find out is to try it. Even if they are able to live together after you get through integration, the integration process can be really challenging,
What are the ages, basically are they all mature or are some still immature? That will affect their behaviors between the two males and between males and females. If some are immature that could lead to bullying, possibly male to male but more likely mature hens toward the immature male and females. (I see you posted while I was typing. They may be about the same age but it sounds like some are more mature than others, more pullets and cockerel rather than hens and rooster.)
If they are all mature, the males will determine which is the boss. That pretty much means a fight. It could be a fight to the death, it could be mostly running away and chasing. Roosters can die or be seriously injured or they can reach an accommodation on how to work together to take care of the flock. How much room they have is often critical. The loser of the fight needs enough room to run away and get away. While there can be exceptions, a typical accommodation is that once they determine who is boss they each set up a territory with their own harem. If these territories are out of line of sight of each other you have a much better chance of success than if they can see each other. as I said there can be exceptions, I have seen two males hang together with a flock of hens in a free ranging situation. They knew who was boss, occasionally there may be a short skirmish, but overall it was pretty peaceful. But that is rare and thy were not squeezed into a tiny space. I consider room critical.
Once that cockerel matures you can try, but be ready to intervene. Anther option is to permanently set up a separate coop/run and keep them separated forever, probably with their own females.
I don't know your goals for having chickens and especially for having roosters. The only reason you need even one rooster is if you want fertile eggs. Everything else is personal preference. Personal preference can be very strong, you need to go with yours and not mine. I suggest people keep the minimum number of roosters they can and still meet their goals. That's not because you are guaranteed more problems wit more roosters, but that problems are more likely. Many people keep spare roosters for a breeding program, for example, but they have to find a way to keep them that works for them.
I see you posted again. Good, more information. It sounds more and more like different levels of maturity between the two groups. I strongly believe it is better to let them mingle when they have plenty of room instead of where space is tight. Often that's all you have to do, give them room and time to work it out. But it is often a good idea to house them side by side for a while to let them get used to each other, a week or two is usually enough for that. It can be a good idea for them to sleep in separate locations even after they are roaming together during the day, mine are most brutal to each other as they are settling in for the night. Don't rush them. They are free ranging. I don't know if there are limits to that but most of the benefits of adding clutter to a run should not apply. Still, separate feeding and watering stations may be beneficial.
As I said, your goals, desires, and decision. But the more I read the stronger I'd suggest you sell him if you can and wait to add another rooster if you really want one.
