Thank you everybody for the great input. I especially like MartinTheMess tower design that combines breeding, iron farm and other things into one tower, thanks for providing the download.
Do wood trapdoors count as "village doors"?
Do iron doors/trapdoors count as "village doors"? (I think the answer is no)
SUMMARY OF VILLAGER KNOW-HOW:
For anyone else finding this thread I summarize what I learned from this thread and others. Please correct me if I misstated something.
The game calculates the center position of the village based on the positions of the doors. If your "village" is an iron farm cell or anything else symmetrical, it is easy to figure out the center.
For most iron farm, breeder, crop farm designs the safe distance between these "villages" is 64 blocks from their centers. Some people go to 72 blocks for extra safety. The distance is measured in any direction as a sphere, that includes up/down.
If you place any doors (and I think trapdoors too) inside that 64 block radius you WILL mess up your setup as the village radius and center will change.
Villagers will breed until the villager count in the village reaches 1/3 of the number of doors (0.35 to be exact), assuming they have food or a field they can harvest for food.
Assuming I understood correctly, separate from the village size, villagers are "in the village" up to 5 blocks vertically from the village center. This is how breeder cells are constructed (top cell with 6 doors defining a village and one villager to keep it active, breeding pad with a crop and 2 villagers to start 6 blocks down. Since they are not counted "in the village" but are inside the village radius they keep breeding indefinitely - until population is too high for area and they start suffocating against the walls or the children are funneled away).
If I understood correctly there is a maximum of 200 villagers per village. Important for your trading hall: After selecting which to keep, destroy the bad villagers and funnel the "good but not awesome" villagers away to a holding area at least 100 blocks away.
There is a great mod that will show you the borders of your village: Village Marker Mod. It works in single player, and can work in multiplayer ***if the server has the right plug-in***. It is based on LiteLoader, so you likely end up with a fairly typical Forge+LiteLoader setup for your client mods.