I can close off the rooster’s sleeping area at night so he does not have access to the girls.
It seems the older hen is ok with the sexlinks and doesn’t go beyond establishing the pecking order
How does that rooster behave around the younger ones when free ranging? This implies he is not the problem. I never have a problem with a mature rooster around young pullets. Each chicken is an individual and we all have different set-ups and management techniques so we can get different results. Roosters can cause some issues but in the kind of room that free ranging implies I just don't find them toward young pullets. Tight space can cause problems for any chickens.
everyone tries to excessively beat up on the brahmas
Can you post photos of the Brahmas, maybe we can tell you if they are boys or girls? A close-up of the head so we can see combs and wattles, if any, and a shot showing profile, posture and the legs can be helpful. 10 weeks is pretty young for the boys to be getting any special attention from the rooster or the hen and Brahmas are often pretty slow to mature anyway. What does that excessively beating up the Brahmas look like? When does it happen? With enough room I'd think it should not be happening so I'm curious as to what is going on.
My question is when can I place the younger ones in the coop as well?
A lot of this depends on what your coop looks like. How big is the coop in feet or meters, how are the roosts laid out, and how cluttered is it? Clutter means places to hide under, behind, or above. How long have they been free ranging together?
I practically always have juveniles in the flock, sometimes as many as three different aged juveniles with the adults. They tend to form sub-flocks, each age group staying together and avoiding the others. Well except the adults, they pretty much go wherever they please and it's up to the others to avoid them.
I have a big coop with clutter. After my chicks have free ranged with the adults and others without issues for a couple of weeks I put them in the main coop with the others after it is pretty dark so the others can't attack them. Then I'm down there at the first crack of light to see how it is going so I can open the pop door and let them escape if they need to. With mine and my coop that has never been an issue but I still check for a couple of mornings. I don't know how big your coop is or how it is set up, you may have issues. With your small numbers of chickens you may not have much room.