The orks do not follow Chaos, for Chaos cannot offer what the Orks want that Gork & Mork already couldn't. All they need is a good fight. Chaos could offer them a good fight, but they already do that themselves anyway, so why bother?
Furthermore, anything Chaos related isn't considered 'Orky'. In fact, it's seen as weak within society. As such, no Chaos Orks exist. Though i wouldn't be surprised if some had a good liking to Khorne.
Furthermore, they are easily manipulated. Whereas a human's mind slowly succumbs to the temptation of Chaos before being able to easily control, a single Ork warchief simply needs to be 'set up' to do as the Chaos deity wills. The first Dawn of War and the WH40k Space Marine games have a storyline like that (Ork invasion set up by Chaos) and i believe that the Armageddon wars were used to 'weaken' the strength of Man to make for easier capture by the Chaos forces.
Chaos Eldar = Dark Eldar.
Tyranids aren't prone to the manipulation of Chaos, as the Tyranid swarm is a hive-mind collective of no to barely any free-will. If the Tyranid Hive Mind that controls it succumbs to Chaos, we'll see. But i dont think even the Gods know where the Tyranid Hive Mind is located.
The Tau have, thus far, no connection to Chaos. Not only is the mention of Chaos barely known to such a young species that's on the other end of the galaxy, they do not use the warp for space travel. They do not possess psychic abilities, so there's no 'patient zero/dark messiah/etc. etc.' the Chaos Gods can call upon, so they cannot establish anything of a foothold.
The Tau do have 'Ethereals', which are sort-of-leaders of the Tau. They have something of 'magic' upon them, as they apparently have a soothing, calming, convincing and inspiring aura upon them (and are, as such, great leaders), but no connection the warp whatsoever.
The Necrons have their own 'dark deity', the C'tan and are stripped of the weakness of flesh 'in spirit and body' and are, as such, incorruptible.