Yes, all the ASK's work with any size iPad Pro, but practicality is another question. Have used a 12.9" ASK with a 9.7" iPad Pro, and a 9.7" with both a 10.5" and 12.9" - all worked perfectly but were rather comical.
Although, if you're using a 12.9" ASK as a sort of stationary "dock" that might actually be a decent thing, even with the smaller Pros