Chaos Tau are an army idea that is bounced around often but rarely thought-out or taken seriously. It is a given that the Tau biology does not grant a very powerful warp signature, but in what other respects do the Tau allow their bodies to stop them? Drones handle most manual labor, battlesuits replace Space Marines, and their poor vision is augmented by targeting systems. Why would the warp be any different?

Here's the scenario: During an secret experiment to advance Tau FTL drives to full warp capacity, earth caste scientists accidentally create a stable warp portal in the engine room rather than make a translation. Terrible pink horrors pour out, tearing into the Ethereal and several engineers before the gate is closed, banishing the creatures instantly.

Despite not being corrupted in any way, the experience was enough to drive many of the Tau present to madness. Among them was the project lead, wracked with self-pity and guilt and the Ethereal's death, who walked right past the cadre of black-ops firewarriors guarding the engine room and reactivated the device during the sleep cycle. Something huge and bird-like waded out of the portal, shrugging off the pulse fire that slayed the project lead and making a universal sign of truce. The Lord of Change cried out in a Tau dialect, "Cease fire! For the greater good of all our peoples, please, listen to reason!"

This stopped the firewarriors in their tracks.

In council with the fire caste commander, the Lord of Change spun lies of another dimension beyond the three apparent. He claimed that the form he wore was more akin to the shadows of the Tau than their bodies, and that in his home realm all lifeforms coexisted peacefully in a unified empire with his avian race as the undisputed "more equal" masters. However, the genocidal Imperium was driving them out to seek refuge in three dimensions, so sorry about the mob of panicked refugees earlier, you know how lesser races are.

Every word was carefully chosen over millennium to resonate exactly with the Tau mindset, and the bait was taken as quickly as Tzeentch expected. The Tau aboard swore their loyalty, the pact was sealed. The laughter of thirsting gods echoed in the shadows of the deepest holds.

I have more ideas related to this scenario, if anyone is interested.