You are asking a lot harder question than you realize. The rules of thumb for 4 square feet per chicken in the coop and 10 square feet in the run don't really apply in a tractor. Yoiu compensate for lack of space by additional management. It's all in how you manage your chickens. What you are talking about is a chicken tractor. In Alabama that should work for you year round, but it requires a lot of day to day management. Depending on your conditons, you may want to consider other options. There are different tractor designs that can work for you, but it pretty much requires moving it every day or two to keep them from getting bored, and they can have serious, dangerous behavioral problems if they get bored. Some people have to move their tractors twice a day.
I have trouble keeping more than 4 chickens in a tractor that is easy to move. By the time you build in roost space out of drafts, a couple of nesting boxes, and a way to keep the feed dry, it just gets too big and heavy.
Until I see your proposed design, I really can't comment on specifics. Is it doable? Yes, but it takes a bit of planning and a commitment to daily management. If you have specific plans we might be able to better comment.