Without knowing your timetable it is tough to answer your question. However broiler/fryers are normally fed a special 22% or higher protein feed.
It is not the feed that causes a meat chicken to grow fast, it is the chickens' DNA that causes it. The extra protein is merely needed to support the extra fast growth, not the other way around.
You should be prepared to process all 12 birds at one time. That will result in one chicken in the freezer per week for 3 months. This will allow you to start a new flock every 3 months, resulting in "sustainable" chicken dinners.
Cross bred meat birds generally do poorly free ranging. They grow too fast to move about freely. If you succeed in free ranging them the meat can tend to be stringy, dry, or tough. Good luck!