You have to do it “the other way around”:

Add all the staves to all the measures in the entire document.

Then, in Page View hide the empty staves where you do not need them.

This {hiding of empty staves} is done with the Staff Tool.

The hidden staves will still be visible everywhere in Scroll View.

Another question then,  if I have a lot of room at the bottom of a page how can I re-position the staffs to fill the space... I did it once and now I can't remember how...

You can move the staves with the Staff tool. Grab the handle, and drag up or down.

 https://usermanuals.finalemusic.com/Finale2012Mac/Content/Finale/Tut7Formatting2.htm 

You can also use the Staff Tool's Respace Staves option.