Did you have any issues with the hens or rooster disturbing or pecking at the pullets?I'm with LG. A few years ago, many of us learned about the heating pad system of brooding and took it a step further and began brooding right outside in coop and run since the heating pad cave so closely mimics a broody hen.
So, in answer to your question, you do not ever have to brood in a container indoors. In fact, after I did it for the first time, I saw so many benefits I never realized were possible, I will never go back to putting baby chicks through the oppressive confinement of being brooded in a box under a hot light.
I wrote up an article about brooding outdoors linked below.
I have a three stage set up right now and it's kinda driving me crazy haha. One for pullets indoors under a lamp, one transition outdoor coop for my middle stage birds, and then a large outdoor coop, run and free range pasture for my adult birds.
Just have been worried about transitioning young birds to my large flock as I have some hens that can be nasty.