in colder climates, most of us put an insert into our screened bottom boards in winter. if you look at it, it should have grooves for sliding a thin board or piece of metal in. mine come from Mann Lake with the insert.
a lot of people use all one size box. it's easy for frame switches. easier on the back.. you just put as many on as you need and there is no problem with the brood area. take off what you don't need for winter.
if the bees have stored enough, then you are correct. often they don't, or we do not calculate properly what we can take and what should be left. most bees starve, if they are going to starve, at this time of the year and going into spring.
feeding pollen depends on your area. i have very early pollen, so i don't need to supplement. i have very wet springs, so if i want to encourage early buildup for excess honey later, i need to start feeding now and get that queen laying. + my springs are unpredictable, and better to feed than have them starve in a cold and wet spring.
that's just general info. hopefully someone from your area can help you with specifics.