In the live letter, it was said that new housing zones will only be added once existing ones are filled to a certain percentage.

What this rule is saying, is that there will be FCs forced to buy the smallest plots of land even if they have money for bigger plots, because otherwise new zones simply won't open and bigger plots simply don't exist.

Could the devs clarify on this?

In 2.1, will we be stuck with small plots that nobody want (since personal housing is not there yet) and new zones simply don't open because there are still small plots open?

And a related question: will personal housing get its own zones in which nobody may purchase plots as an FC, or will personal housing be mixed into 2.1's FC housing zones, so that any individual who wants a large plot must compete with every other FC out there?

TL;DR: question for devs:
  1. Will personal housing be mixed into FC housing zones?
  2. Will individual players ever be competing against FCs for land plots?
  3. In 2.1, will we be stuck with no new housing zones because existing zones are full of small empty plots that nobody wants?