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They are aware that the Warp exists. That's about it for most of the rank and file. Higher order Ethereals and commanders are aware of the corrupting and mad nature of the Warp, and ordered research on the subject to come to a full stop after Medusa V, and witnessing the fate of the Earth Caste researchers as the planet fell to the Warp. Aside from that, it will vary from commander to commander and world to world. Tau stationed on the Damocles Gulf are more likely to encounter Chaos forces and simply accept that "These things are weird, and this is how they opperate" while Tau in the inner worlds of the Empire are less likely to be aware of Chaos at all.

i had another Tau thought.

the Imperium has stopped advancing. the tech they have is all the tech they are going to get. but the Tau are always upgrading.
inevitable one of four things will happen

1. the Tau will outstretch the Imperium by enough that they will conquer it all
2. the Imperium will get a big enough break from outside incursions to launch another crusade that will overwhelm the Tau numerically before there weapons are that good.
3. the tau will get psykers and have an end to there age of advancement
4. the Imperium will have a reformation and won't be the imperium as we know it anymore.

basically on a long enough time line if the status quo continues the Tau will defeat humanity.

any thoughts or objections?

There are a lot of problems with this line of thought. One, the Imperium does advance technically, just at a glacial pace, as they're dependent on salvaging STC's from the Dark Age of Technology. Two, this completely ignores the presence of all other threats that could conceivably wipe out the Tau; Hive Fleets Kraken and Behemoth, the Orks, and of course, the Necrons. Three, the Tau already have psyker allies, and the chances of the Tau gaining psykers within their race are minimal.

The biggest problem the Tau will face in the future is communication. They don't have an astropathic network, or any real means of FTL communication. This hasn't been much of a problem at this point because the closeness of their worlds means that courier ships are still viable, but once they start expanding outside of their cluster, communications will break down.

None of the other races have stagnant tech save maybe Eldar. Orks have what they can salvage from other races, and while they can always build shootas, as long as someone is advancing, so are the Orks. Furthermore, if the grayskins become a better challenge, that means more Orks will be drawn to fight them. Eldar aren't advancing in techbase, but they are always going to outstrip the Tau in sheer psychic power. However, their supposed threat to the Tau seems minimal, seeing as Eldrad Uthran himself (the d**k) sees hope for the galaxy in them. Tyranids are constantly evolving and changing, so they're essentially the opposite of stagnant. Necrons... well, they have the guns they have for the moment. The Necrons we see on the tabletop right now aren't warriors, they're the Necron equivalent of farmers. The Gauss Flayer isn't so much a gun as an energy collecting device to feed C'tan. We haven't seen true Necron warmachines yet.

Again, highly, highly unlikely. For starters, aside from some of military, Space Marines, and Inquisitors, the majority of the populace of the Imperium is completely unaware of the existence of the Tau Empire. Aside from a few border worlds where the Tau are actively trading and promoting the Greater Good as an ideology, the Tau are unknown. So that in itself is going to keep any significant portion of the Imperium from trying to secede.

Also, the citizens of the Imperium, by and large, are told from a very early age never to trust the words of the heretic, the mutant, the xeno. Obviously, all the Tau's pretty words and flowery promises are lies, right? In addition, if a world was to actively secede, the response would be swift in coming against the world. Probably not Exterminatus, but a brutal attack force to put the planet under martial law. Possibly orbital bombardment of the hives.

In any case, regardless of demographics of the Empire, the Tau are 'more equal' than any other race in it.

As he said, against all odds it cant happen. Thats why the imperium polices information first. You cant allie with what youre unaware of.

If it were to happen, most would face leadership changes. The Ordo Xenos and the Offcio Assinorum would take out the governors or leaders of any group of protesters or traitors. Pro Imperial (most likely retired gaurd) leadership would be put in place and stricter reign on the populace enforced.

For those that muster a defense you have military, navy, bombardment, exterminatus, and if any marines catch wind of this, or reply to a military request you know some pplanets will feel some hurt.

Anyways, you persist, but the thing is, what the tau do would be based on what the new demographic or major minority does. Do they simply protest? Rebel or riot? Try and oust the tau government? You need to set up more for this ypothetical situation, which will be hard as there isnt a lot of fluff on the lives of the tau empire humans from what Ive seen.

One huge thing...well more like...Five big things. This isn't a one on one war, so the empire wouldn't be able to send everything at them, at risk of being over ran by orks, nids, chaos, eldar, and dark eldar. Beisides lets remember where the tau empire is, right next to Ultramar. So the empire would only send the Ultramarines, and (maybe) some imperal gaurd regements. That is because I find that the empire can be ,at sometimes, very egotistical. So they wouldn't send much to fight the Xenos, Then again they are zealots of xenophobia. *rubs head* Ack too many other ideas, (like the ultra turning the tau because of their closeness to them). See this is why I've started an ork army, less politics and more WAAAGH!

edit: Oddly enough this did happen! well.... for one world. The world Moracre (closest imperial planet to the tau empire) did plege its alliance to the tau empire. Sadly the rebellion was stopped by the imperium.