Hi all!
I have an 08 Stumpjumper and a 96 Kona Cinder Cone. The drive train on the Stumpy is overdue a replacement and I started collecting parts to replace it all a while ago.
I was idly thinking today about converting both to 1x systems as I rarely use the big ring and never use the granny ring. In fact i removed the granny ring from the Kona about 10 years ago, I guess that makes me a trend setter!

So as far as I can see i need:

Kona: 1×9
11-32 cassette (got a new one already)
32t ring for middleburn RS2 c/set
chain (got)
9 speed shifter (got on Stumpy)
single ring c/set bolts
chain device for 28.6 seat tube

Stumpjumper: 1×10
11-36 10 speed cassette
10 sp chain
34t ring (hollowtech 2)
10 sp shifter
single ring c/set bolts
chain device (direct mount)

So, my questions are:
Will a 9 speed cassette fit on an 8 speed freehub body?

Will i need a 10 speed rear mech for my Stumpy? Would i be better buying a clutch mech/thick thin ring than using a chain device?

Do the gear ratio’s look ok? I currently use a 34t/11-30 cassette on the Kona and rarely struggle on climbs ( I just climb out of the saddle.) So should I run a 32t/11-36 on the stumpjumper?

What chain devices are good for XC stuff?

If you have stayed awake so far, well done!

Will a 9 speed cassette fit on an 8 speed freehub body?

Yes

Will i need a 10 speed rear mech for my Stumpy? Would i be better buying a clutch mech/thick thin ring than using a chain device?

10sp Mech for 10sp Cassette
2nd part depends, something like a e-13 device with lower bash ring will protect you from striking the ring. Though many report thick/thin & clutch work well too

Do the gear ratio’s look ok? I currently use a 34t/11-30 cassette on the Kona and rarely struggle on climbs ( I just climb out of the saddle.) So should I run a 32t/11-36 on the stumpjumper?

Depends entirely on you, your legs and where you ride. From my point of view they look a bit low but if you can spin very fast then it will probably work fine.

Cheers, any idea the difference between 9 and 10 speed mechs? It’s a sram X0 thats on it.

I guess the gearing is all subjective. I tend not to spin gears and prefer to stand up and push a bigger gear at lower rpm if I get tired sitting. All riding is in Scottish highland, mostly uppy/downy swoopy singletrack. Might try a 32 as I already have one and see how I get on!

I’t pull ratios that make the difference, move your 9sp to the Kona and get new 10sp. If you want clutch then you have to go 10 sp.