Will he harm them over time?
You do not get guarantees with living animals, it just doesn't work that way. Part of the mating process is that the hen squats. That gets her body onto the ground. That way the rooster's weight is passed into the ground through her body, not through her legs. That tremendously reduces the pressure on her from his weight. Roosters of about any breed re heavier than hens of that breed. The squat is nature's way to protect the hens. The more difference in weight there is the greater the chance of injury, but many people keep bantam hens in with full sized fowl roosters without problems. I can't give you any guarantees but I personally would not worry about it that much.
I think he could drop his sneak-attack method if he had a flock of his own.
A huge unknown. When you have two males of a certain age in the flock, they know which one is dominant. The dominant one may suppress some of the behavior if the other or the competition may spur either one on to less than acceptable behaviors. It's hard to tell how a non-dominant male will behave if he suddenly becomes the dominant chicken in the flock. There are a lot of dynamics going on between those two males and between them and the girls. Lots of different things could happen. Certainly no guarantees here.
What are your goals related to those boys? The only reason you need a rooster is if you want fertile eggs. Anything else is personal preference. Nothing wrong with personal preference I have a few myself. But those are a want, not a need. My general recommendation is to keep as few males as you can and meet your goals. That's not because you are guaranteed more problems with more boys but that more problems are more likely. I don't know what the right answer is for you, that depends n your goals and desires.
Sometimes two roosters can coexist in a flock, especially after they leave puberty and become mature. There are different ways that can work out, a very common one is that they have enough room so they each claim their own territory out of sight of each other. The number of hens each has isn't that important, one member on here has several roosters, each widely spread out in their own territory, including a separate coop to sleep in, with each rooster only having 2 or 3 hens. Another does that with maybe 7 hens or less per rooster. But a huge part of that is that they are mature hens and roosters, not pullets and cockerels going through puberty, which it sounds like you have.
