In practically any breed the rooster is bigger than the hen. Nature did some things to help protect the hen during the mating process. I'll copy something I wrote that describes the why as well as what of the idealized mating between adults.
The rooster dances for a specific hen. He lowers one wing and sort of circles her. This signals his intent.
The hen squats. This gets her body onto the ground so the rooster’s weight goes into the ground through her entire body and not just her legs. That way she can support a much heavier rooster without hurting her joints.
The rooster hops on and grabs the back of her head. The head grab helps him get in the right position to hit the target and helps him to keep his balance, but its major purpose is to tell the hen to raise her tail out of the way to expose the target. A mating will not be successful if she does not raise her tail and expose the target. The head grab is necessary.
The rooster touches vents and hops off. This may be over in the blink of an eye or it may take a few seconds. But when this is over the rooster’s part is done.
The hen then stands up, fluffs up, and shakes. This fluffy shake gets the sperm into a special container inside the hen near where the egg starts its internal journey through her internal egg making factory.
Not all adult matings go this way and you are not dealing with adults. Also nature does not deal with absolutes. She may make things safer but that doesn't necessarily mean they are totally safe. Also breeds are manmade. A feral flock in nature would not have a huge difference in size, not like different manmade breeds can.
As others mentioned they have big differences in size yet do not have problems. But that doesn't mean nobody ever has problems, as you can see. The hen squatting to get the rooster's weight in the ground through her body instead of through her legs does help a lot, as long as she squats. The more difference in weight between the hen and rooster the higher the risk.
Another big part of this is technique. Mature hens generally know to squat. Mature roosters generally have learned a better technique and usually have the respect of the hen so the hen is more willing to cooperate. Again, that is not absolute. Not all mature hens cooperate with all mature rosters. Some mature roosters rely more on force than personality. But with most mature flocks there is little drama.
Not all immature pullets cooperate, especially with immature cockerels. Cockerels tend to rely more on brute strength than personality. They are generally bigger and stronger than the pullets and hens and let's face it, their personalities hopped up on those hormones are generally not that appealing to the pullets and hens.
The difference in size between my cockerels and pullets or hens and roosters are not as dramatic as yours. My pullets are not crippled. Some pullets may lose a few feathers, especially around the back of the neck or on the back, but I've never had one ripped like you describe. I think the difference in size and maturity levels have a lot to do with what you are seeing. If you keep them separated as you plan they may be OK together once they all mature. Maybe, but I can't give you any guarantees with that. At some point you may be forced to decide between your little girls or big boys. I try to decide for the benefit of the flock as a whole, not for any one specific member. To me I'm keeping a flock, the individual members of that flock can change.
Good luck!
