Shaaki, if you hatch eggs, you are going to get some roosters. Do you have a plan for that? Obviously your eggs won’t be fertile, but just in case you consider getting fertile eggs. I suspect those are some type of red sex link hens, a cross instead of a specific breed. You should like them.
Chatty Kathy, you probably can’t if that is vicious pecking. Some hens will adopt chicks about any age, but most imprint on their chicks after hatch. With most you can introduce young chicks and they will adopt them, but the older the chicks get the less likely that is. Part of that is the hen rejecting the chicks, part is the chicks not accepting the hen as their mother. They imprint too.
I don’t know how vicious that pecking was. It’s possible instead of the hen trying to kill them or run them away, she was trying to get them to listen to her and accept her orders. It’s hard to know exactly what is going on over the internet. I don’t know if the problem is the hen or the chicks or whether it would work out.
Sticking them under the hen at night is usually the best but I’ve had some that I gave the chicks in the middle of the day and they immediately take them over. Those chicks were fresh out of the incubator and the same age as her chicks. At the age yours are, I'd be really nervous about trying that.
They are living animals and not always consistent.