I don't see any bantams in your mix. That helps with a Brahma male.
When a mating takes place, the hen squats. She actually puts her body on the ground. That allows the weight of the rooster to go into the ground through her body, not just through her legs. Stress is measured in pounds per square inch. By greatly increasing the number of square inches you greatly decrease the stress. In a lot of breeds the rooster is a lot bigger than the hen, but by her putting her body on the ground it doesn't matter.
There are people that have some bantam hens in a flock with a full-sized rooster and don't have issues. The more difference in weight the greater the risk, but that comes in more with whether the hen squats and the rooster's technique than just pure size.
You can always have exceptions when you deal with living animals. That's what makes it so hard to predict what will actually happen. Practically all mature hens will squat for a mature rooster, that's in their nature. With mature full-sized hens and a mature full-sized rooster I personally would not worry. The larger bantams would probably be OK even with a Brahma but I'd avoid the tiny bantams.
You don't have mature hens and a mature rooster. You have immature pullets and an immature cockerel but at least they are all full-sized chickens. Most of the time, even when the pullets resist mating, they will squat when the cockerel forces them. That's an instinct to protect them. I'd worry more about the violence of him catching them than the actual mating. Usually that is not that dangerous. When he grabs hold of them they typically submit. But the cockerel has probably not developed a great technique and some pullets might not submit. When you have violence there is a risk.
Your question is will he be OK with them when they are all mature. In my opinion, yes. They will be fine with those breeds and mixes. What you did not ask is what about now. Personally I let mine stay together as they mature. It can be hard for some people to watch but I consider it part of the growing up with the flock experience.
I typically have 40 or more cockerels and pullets of various ages growing up with my adults in the summer. Sometimes I have a lot more cockerels than pullets. That doesn't bother me. But once every three or four years it gets rowdy enough down there that I lock about a dozen cockerels up until they grow to butcher size. Yes, I consider it a good thing to have a place ready to put one or or more chickens on short notice. I don't automatically lock my cockerels up because of what might possibly happen, I base what I do on what I observe.
You can isolate that cockerel now if you wish, by himself or with some of the larger pullets, then integrate him when they are all more mature. Or you can just observe. Have a place ready if you feel you need it.
Good luck!
