I was afraid of that when you explained your set up to me. Being in plain sight is not the same as interacting with other chickens, interactions determine pecking order. If only one bird attacked the pullet, pull that bird and another bird, put the pair in your broody pen and keep them there for two weeks. Once you pull the meanies, immediately add the broody hen and chicks to the others. This will allow you to get the broody hen and chicks into the flock and established while the hormones of the broody hen are still active. By pulling the aggressive birds, they come down in the pecking order. By taking a pair, it will make the eventual integration easier, two birds is easier to integrate than a single bird. Basically when you added back the broody, it was adding a single bird, hence the attack. What might happen is the other layers attack the broody, but if the broody puffs up and growls they will generally back down. If so, I would leave it, just check them several times today.
Do you have hideouts in the run? A pallet up just high enough that the chicks can get under it can help. The ability for birds to get away is crucial in an integration. You mentioned the ramp problem, my broody tonight went in the coop and created a new nest on the floor of my coop. All of my broody hens have done this. By the time chicks are two weeks old, they should be very mobile, but perhaps if you have kept them in a small pen, they are not. However, if you let them out, they will quickly get more athletic. So whereas you may have to go down a night or two, just a little earlier than the layers go to bed and just make sure everyone gets into the coop, by 3 weeks, my broody hen will have hers roosting on a board several feet off the ground.
If this does not work, and it might not, you may have to keep the the chicks separate from the flock until they are very close to 4 months old. This is the problem of separating birds, the flock equates strangers with danger and go into attack mode. I only separate birds that I am going to cull, as separating birds cause more problems than it solves. However once you have them separate, then you have to wait until they are much bigger.
My own broody started hatching Friday night, I went down to the coup early Saturday morning, to hear peeping but she was still on the nest. Next time I went down she had two chicks out of the nest, with her in the run. There was peeping in the nest, 5 downy chicks not quite brave enough to make the leap. I scooped them up and put them near the broody and they ran quickly and got a warm up. I went down several times yesterday, I saw the layers come in to see just what had Butter done. I saw a layer get a drink, and a chick run under her feet, and she ignored the chick. I did let the layers out, just to give her a bit more space, but late afternoon, all were penned up again, Broody was puffed up, chicks snuggled under her and peace in the run.
When you hatch out under a broody in the flock, you may lose a chick, but really the healthy vigorous ones will make it, and the advantages of a peaceful flock is what I really want. The number one way of causing stress in your flock is to separate birds and add them back. Many people wind up in your exact same position, all with the best of intentions.
Mrs K
