I believe third-party Scratch support for Serato is still in development; Traktor Scratch support is currently available.
With regards to the SP1, I believe you need a CDJ-2000NXS2 (or XDJ-1000MK2) in order to take advantage of the extended functionality. Though I suppose you could remap the MIDI to fit your needs, but I don't think there's any direct plug-n-play functionality by combining the SP1 with the DJM-900NXS2. @Mark or @Pulse can confirm or refute, however.
My advice would be to save your cash until you get concrete confirmation.
Cheers.