Yes, there is a chance of that. Just like hens have a pecking order, so do roosters. Some roosters will sort it all out with bluffing and ruffled feathers.
Be aware, comb injuries bleed badly. So if you find a scary amount of blood one day, don't panic. Look at the combs, and you may find a tiny little peck-mark. If that's all it is, they're fine.
Depends on the roosters. Game roosters (aka fighting cocks) are known for killing each other. Any other kind of rooster has a very high chance of getting along with other roosters, depending on how much space and how many hens there are. If you have lots of space, they may just choose to stay out-of-sight of each other.
Yes, although I can't see why you'd want two separate flocks that each have two hens.
You could just put one in and watch what happens (be ready to remove if there's a problem.)
You could let them all out to free range together, and watch what happens (that way they meet in a neutral space, not someone's home coop.)
You could put them in adjoining coops, with wire fencing in between so they can get acquainted gradually (look don't touch method)
You could put the cockerel in a wire cage inside the pen with the older hens (variation of the above).
That is also an option.
Obviously, you've got lots of choices!
I personally would try to make a single flock of all six birds, because I prefer to have fewer separate groups, and because it seems that chickens naturally do live in groups larger than two or three. But that's just my thought, and I really don't know what will work best for YOU and for YOUR birds. It might be that the older hens are content the way things are, and may not want change