Based on my experience, the a medium cage derailleur will be no problem. I just built an MCR as a 2x9 with using a standard XT triple crank set from which I removed the big ring and replaced it with a Race Face 34t bash guard. I'm running a medium cage Sram X9 rear derailleur and cut the chain to eliminate slack in the small ring, small cog position. No shifting issues at all.
Finally did my dreamsicle (dreamcycle) inspired build I've been toying with for about a year. Originally I originally intended to use the Nashbar Signature frame, but I'm stoked I waited long enough to pick up my MCR. Love it!
The white and orange is a bit obnoxious, and I love it!
Waiting for my new Jagwire Hydralic white hoses to come in so I can go all white for the hoses.
Sorry for the pic quality, I'm not the best photographer yet.
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