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.

What is the motor shaft diameter required for the motor pulley, 8 or 10mm?

The motor shaft diameter is 10mm internal but you would use an 8mm inner diameter on your motor pulley.