I was wondering or just plain stupid if prop spiners have to be modified to allow slots for the prop. Do prop spinners require a separate back plate are they sold seperate or do they come with the spinner?

The back plate comes with the spinner.

And yes, most times you need to enlarge the prop holes. Spinners can go on many different size engines, so the MFGers usually cut the holes to accommodate the smallest prop sizes. If you're using something larger, you need to file the hole bigger (A dremel is the tool of choice for this job)

...or you can buy a Tru Turn.