Going to run a 32 up front with an 11-36 rear, maybe an 11-34. What length SLX mech would be required?

A medium GS cage.

Or short cage will be fine, they are ok on 11-40t cassette on a bike with less than 25mm of chain growth. If you’re going up to 11-42t then I’d run a medium cage.

Medium or short. If you run with a clutch it doesn’t really matter, though yes medium if you plan to get an extender.

I would add though that when I ran a short Type 2 on my full sus it was making the classic SRAM Type 2 ‘clonking’ sound on compression. I swapped it for a medium thinking short may be stressing it a bit and that was fine. I put the short on the hard tail and it doesn’t clonk as there’s no rear compression. Could be just an issue with that mech though, not that it’s a short cage. SLX of course is different.