An FAQ is about "all-in all-out".
Don't let your enthusiasm to raise Sunshine Chickens be dampened by that :)
1) Each batch of chicks should be in their own ranging area. If you have to harvest partially like daily or weekly, you may do so. You harvest all from that batch. Do not introduce a new batch in a ranging area with an older batch.
2) After all chickens had been harvested from a range area, let that area rest and recover. Use organic disinfectant, spray the whole area. After about a week, moisten land, scatter lime and rake the area thoroughly. Let the sun and fresh air work on the land :) You will know when your land had recovered. It will look fresh.
3) If you have several batches in different ranging areas, it is ideal that you have different caretakers for each. If you must have one person feed all, start feeding the youngest batch and ending with the oldest.
4) Practice biosecurity. It is not expensive nor hard to make disinfectant/footbaths at the entry/exit points of the ranging area.
5) Observe where the wind is coming from. Try to position the ranging areas in such a way that the wind doesn't blow into other ranges. You are trying to minimize the transfer of airborne bacteria.
6) If you want the fenced area all in one side of your farm, you can use have several ranges separated by net walls. Do not use two side by side pens at the same time.
There are no guarantees that strict biosecurity will make a problem free farm. Use best judgement and common sense. Adopt and drop ways, as you see fit to your personal needs.