When chickens mate the hen squats on the ground. That way the rooster's weight goes into the ground through their entire body and not just through their legs. It's something like trying to poke a hole in something with the point of a needle or mashing against it with the side of the needle. It makes a whole lot of difference in how much stress their bodies and legs see as long as they squat. Squatting seems to be a reflex action so they should squat. It's nature's way of protecting them. The more difference their is in their weights the more risk there is, like anything to do with living animals their is always risk, but I would not over-worry about that part of it.
Your pullets could possibly be laying in another few weeks but a couple of months is more likely.
A good mature rooster should not be mating those pullets. They are not showing any signs of being ready to lay, like bright red combs and wattles. Not all mature roosters are good in everything unfortunately. But there could be another explanation. Those pullets are at an awkward age for him. They are pretty old for him to consider them his chicks but too young to be sexually mature. If they had been introduced to him when they were younger that would just be a part of the flock and of no great interest. He and the hens would probably be ignoring them. It looks like they have a lot of room. Does this behavior occur when they are outside with a lot of room or only when they are more confined?
The mating act is not just about sex, it is a show of dominance. The one on bottom is accepting the dominance of the one on top, either willingly or by force. At that age I'd expect it is mostly by force. I assume it's just the pullets, it could be the cockerel too. The rooster will want to dominate them all. It's possible after he mates with them a few times and established dominance he'll stop until they mature.
That still would feel strange to me but maybe. My mature roosters leave the pullets alone until they mature, say when they start laying eggs. But mine are raised with the flock. Maybe that makes the difference. I'd expect that behavior from your young cockerel as he matures rather from a mature rooster.
That brings up another point, at some time your cockerel will probably hit the point of puberty where he starts wildly mating those pullets if he isn't already. And he may eventually challenge the rooster for flock dominance. You have his journey through puberty to go through. Your type of comments are a lot more typical with a flock that has immature cockerels in it than a mature rooster.
I haven't seen your specific situation and I typically raise pullets and cockerels with my flock of a mature rooster and hens every year. The big difference is that mine are in there with the flock at a very young age. Typically my rooster and hens ignore the juveniles as they go through puberty, including the cockerels chasing and mating pullets that don't want to be mated. The only time the rooster gets involve is when a cockerel starts bothering his mature hens.
I'd probably let it go as long as no one is getting physically hurt. See if they can work it out. But observe and be ready to intervene if one does get injured. Integration, especially with juveniles, can be stressful.
Good luck!
