Now I'm imagining a secret Tau-backed Red Army Faction accidentally discovering a fledgeling genestealer cult and having to wage a clandestine war while dodging the authorities.

This single sentence is more interesting and has more potential than 99% of the recent GW warzones/scenarios that boil down to Named Character X faces off against Named Space Marine Y. Tau-backed insurgents having to fight genestealers while both are being hunted by local Imperium forces? Now that sounds like a sick scenario, hell if GW wasn't afraid of making boxes that don't include an imperium side Tau Gue'vesa vs Genestealer Cult would make a hell of a killteam box

Tau are afaik smaller and lighter than humans they have also have worse eyesight and reflexes than humans....which mind you doesnt matter often since Tau usually have much more powerful weaponry. But you are right that humans probably could act as shock troops within the Tau forces for city combat as an example.

Humans basically join the fire cast I think.