I was planning a gradual upgrade to 11 speed, can I use a DA 9000 rear derailleur with DA 7900 10speed shifters and 10 speed wheels. I understand the derailleur pull ratio has been matched to the DA 9000 shifters, but I don't think that would be much of a problem as the shifter type controls the pull ratio.

Just wanted to confirm before swapping out the rear derailleur, thanks for any advice.

The shifter type determines its pull ratio. The derailleur is intended for a particular pull ratio, and for the most part won't work without it.
9000 derailleurs and 7900 shifters will not work together.

I've seen people mix 9 and 10 speeds. But I think you're better off getting matching sets than experimenting.

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Thats because 9 and 10 speed shifters share the same pull ratio, where as 11 speed shifters pull a different amount of cable. The anchor point of the RD is different as well leading to no compatibility.