Will this work on a 1×9 set up with a short cage mech.

Nope, not without a bodge.

Designed for 10 speed, remove the 17T cog if using Shimano XT mech.

Should work if you can tweak the spacing, might still work without:

9 speed spacers = 2.56mm
10 speed spacers = 2.35mm

The t Rex looks to have a wide base from what I can tell effectively integrating that spacer, so you need to find a way to add ~0.2mm, time to break out the tin snips and old coke cans methinks…