I just sent one child to university in a completely empty town. The game had to spawn the 11 other dormies for her, as I use blank neighborhood templates.
If I sent another child to this university but in a different dorm, how will the game get dormies for that one?
Create new ones, or pull from the first dorm?

Create new ones.

Will it still create new ones, if I move the sim out of the first dorm?

For your second scenario, as long as there are the same or less number of dorm rooms the game should reuse your existing dormies. At least that's how it's been for me with empty templates installed.