If I order multiple single bolt on motor mount kits is it possible to run 2 of them on a setup

Yes, you would need two sets of these to run a dual motor setup.

What's the smallest wheel you could use with this setup? (Seeing as the wheel mount has a fixed size, this seams like a important number to list)

We recommend only 80-83mm Longboard Wheels as the smallest option to use. Typically, we recommend 90mm or 97mm Longboard Wheels as they help for rougher streets.

will this mount work for a motor with 44mm spaced bolt holes?

Yes, it uses a 44mm diameter placement for the bolt holes. Uses M4 Bolts.

Will this mount work with 100mm wheels?

Yes, this will work with 100mm wheels.