If my base is empty, is it possible to place a Special Forces unit next to my empty base (instead of in the base)? It seems like this is possible and maybe even required since Special Forces draw supply diagonally.
Reread the definition of "supply" on page 8.
If a unit draws supply diagonally, that means that (if you don't place it directly on your base) it must be placed so that it's diagonally adjacent to a supplied unit. An empty space (whether it's your base or not) is not a supplied unit.