Heck why not team up its the perfect team because Chaos doesn't give a **** about the Tau and the Tau don't want to go anywhere near Chaos what better alliance is there when both sides don't want to see each others faces tau / chaos armor can't tell me that doesn't make the Fire Warrior look way more Badass
There's a problem. Majority of Chaos marines are on the other side of the galaxy from the Tau Empire. Quite literally. Between us and you, lies hordes of Orkz, Tyranids, craftworlds of Eldars, Dark Eldar raiders, and of course hundreds upon hundreds of imperial worlds, potentially millions of imperial fleets. Oh and the Space marines. There isn't really any real confrontations between Tau and Chaos (apart from DoW) that I know of. Infact I don't even think Chaos even know of their existence (But then again, I could be wrong...) . Even so, me being a word bearer, I believe no Xeno is worthy of worshiping Chaos. Especially one that can't be corrupted, because you would therefore be insulting the ruinous powers and everything Chaos stands for.
While it's certainly possible that elements of the tau military could be corrupted, and they would need to be corrupted to side with Chaos, it would be extremely difficult. Even overlooking their extremely low presence in the Warp and mental resistance of the Aun, you run into the cultural issues. The issue which dealt with the Tau Fire Caste running into Chaos for the first time saw it as the very thing their society had been pressing to overcome, the Mona'Tu. It is the very thing they fear and dispise the most, even more so than perhaps craftworld eldar and Chaos. Convincing any large number of them to join that, even truly breaking away from the rule of the Aun would be extremely difficult.
The Tau would still do it if they really needed,if it really benefited the greater good,or if the hunter cadre/Ethereal in question was unaware of the nature of Chaos. I mean the Tau "teamed up" with the DE once. Its still a possible aliance,but so,so very unlikelly.
Tau beings are well known to not have a proper soul\psychic projection in the warp. They're both immune to the Chaos Gods and the Warp stuff. Speaking about soulstuff, they strike good similarities with the Necrons. They do not even use the Warp to space travel. So basically this won't work at all. Yet, everyone is welcome to pledge allegiance and servitude to our Dark Gods, but doing so after being literally immune to the Warp, both curse and gift-speaking, makes this actually pointless.
Nonsense, the Tau are not immune to the Warp. The fact that they register as small specks within the same does not make them immune to the destructive powers themselves. Its like this, imagine the Warp is a car. The Tau cant drive this car but they can be killed by someone who drives over them with it.
Dark Eldar are not like Chaos Marines in anyway. I mean Dark Eldar are arrogant, overconfident, devious, while Chaos marines are barbaric, insane enough to serve cruel beings that are the Dark Gods, and being adpetus astartes wish to purge all xenos, including Tau. Infact Dark Eldar despise Chaos Marines for their devotion to Slaneesh (at least most of the Chaos Marines apart from the ones that don't serve any God). Also on that point, I could imagine an alliance between some Chaos Marines possibly Alpha legion, maybe one of the other legions. But not with Emperor's children, Death Guard, Word Bearers, World Eaters, Black legion, Thousand sons or the other fantical worshippers of the Dark Gods. There are some Chaos Marines which are kinda of just rebelled the imperium's rule for the sake of it.
