My Standard Cornish are the exhibition type and are very mellow with people. None of mine have ever made any kind of aggressive move towards me. I just walk in and pick them up, no chasing, no fuss. I love them, they are my chunky cuddle monkeys.
If they weren't raised with the other roosters, I would not put a strange adult rooster in with another. I don't know how much space they would need to be ok with another rooster being around.
My roos are ok at shows, they posture ect. but haven't started any trouble with surrounding roos. At home though with hens they much more territorial with other roos. I don't know if they would try to kill each other, but they are so strong I wouldn't want to test it.
My younsters do not have any trouble staying together. As long as they were raised together, there isn't a problem.
Holding a large cornish is like picking up a bowling ball. I have no experience with the bantams, but standard are Heavy. You don't want to get stuck carrying one around.