I've been experimenting with this a lot, and hoping to have enough information for a blog post on it soon.
For a demiglace equivalent, which will presumably have a healthy concentration of natural gelatin already, I'm using
0.2% lambda carrageenan
0.1% xanthan gum
These both hydrate cold, with shear from any blender.
It's important that you don't thicken until you've thoroughly defatted. Both these hydrocolloids are powerful emulsion stabilizers, and you will get a cloudy, oily emulsion that will never separate if you don't defat first.
Also important that you don't use this in a sauce that contains dairy. Calcium ions with cause it to form a gel.
I'm also experimenting with:
1:1 guar : LBG
3:1 lambda carrageenan : xanthan
propylene glycol alginate
For adjusting thickness of final sauces, the most popular choices are modified starches that are pre-hydrated, like ultrasperse and ultratex. These dissolve easily and don't need cooking to hydrate.
I prefer the clarity and texture of a starch / gum blend, typically
1:10 xanthan : arrowroot
If you mix them together thoroughly as powders, you'll be able to make a slurry without the xanthan turning to clumps. This can be whisked into a sauce, and only needs to be simmered briefly to hydrate the arrowroot.
This starch / gum blend has the advantage over the previous gum blend that it doesn't need blending, so you can adjust by feel right on the stovetop.
