Yeah, I was thinking about that last night. Tau don't have psychic abilities and the presence of Ethereals kinda means no Chaotic mental manipulation. I'm not even sure it would happen if there wasn't an Ethereal around. The Tau would certainly change their thinking (like Farsight) but I'm still not sure Chaos could get a word in.
If they ended up on a Daemon world or were exposed to the Warp itself somehow, then perhaps they could be affected physically, and THEN perhaps mentally as well.
I know I'm deviating from my original post a little now, but it's my post and I can.
So ner.
All of this has made me wonder about the affects of Chaos and the other Xenos races in the galaxy. Which ones could be affected, how would it affect them and which Powers could affect them?
Orks: We know from the older fluff (especially Freebooterz, of which I haz a kopy) that they are reasonably susceptible, though aside from some mutation, you probably wouldn't be able to tell much.
Eldar/Dark Eldar: I know Slaanesh has the monopoly on them, but could the other Powers 'steal' one or two from time to time?
Tau: Tricky one. See above posts.
Tyranids: I figure Slaanesh has no hope here, but the other three big guys might get a shoe in.
Necrons: ++Query: Chaos? End Query++ Clearly, not going to happen.
I know there are various other minor races dotted around and books like Xenology have expanded upon their backgrounds but, even after 20 long years in the hobby, I confess to not knowing enough about them to make a call on them