Depends where you live. 6- or 7-chicken tractors are pretty feasible in a climate that is reasonably warm and mild all year round, although you will spend a lot of time and energy moving the tractors every day or two and you will not save *much* on feed. However, tractors that size are not nearly so doable in the North where winterization is an issue. In a climate that gets actual real winter, you'd be better off iwth a fixed coop and if you want them to have vegetation and bugs then you can get some and toss them into the run every day or two for the chickens to pick over. Again it will not save you *much* on feed bills, but IMO it makes the chickens happier and usually healthier.
Good luck, have fun,
Pat