Hello,
You are correct, if you use Fixed Work and set the Resource Unit% manually, the duration will be the third piece that will be adjusted.
In order to specifiy Availability (Maximum Project Availability), you can do so in the Resources Related Panel (at the Project level). see attached image1.
In order to specify Resource Availability at the Task level (Unit%), you can do so in the Resources Related Panel (at the Task level) see attached image2.
Sincerely,
Boris