I can’t for the life of me work out whether I’d get away with the XT GS rear derailleur (the short one). Total capacity is 35 teeth. What does this mean?
I want to run a standard 1×10 setup with 32 or more likely 34 upfront and an 11-36 cassette. Went for the long cage on another bike and not surprisingly it works fine. Should I do the same again or will the short mech be OK?

GS will work fine. 11-36 with single ring requires a mech with a total capacity (c/ring size difference front+sprocket size difference back) of 25T+, so 35T leaves you more than enough room for chain growth on any FS bike.

Cheers