Tau cannot see the Warp and they don't travel through it. Their ships are FTL capable but that is also why their empire is so small because they rely on technologies that are not as efficient in long distance travel has other races (by the same token this is how the Necrons travel). Nor do they have any kind of psychic power. But as per the fluff of the setting the Tau are taking an interest in the Warp Drives of other races to see if they can make use of it. But they understand the danger the Warp represents and are not ignorant of the Imperium or the Chaos Marines. More often than one may think the Imperium has allied with the Tau, and actually upheld the truce once that conflict was over.
You are right in that the Tau don't travel through the Warp and that's the reason why I would think the Tau might be interested in getting their hands on some Warp tech. In regards to Imperium and xenos its true that the Imperium can ally with aliens if needed no problem, but when the alliance isn't needed its back to "extermination on sight" policy again.
Well the Tau are aware of chaos. They are aware of their followers and daemons. But they are not fully aware of its threat (I'd say 60-70% aware). Mainly because the chaos gods and chaos daemons aren't very interested in the Tau, and the Tau may be slightly resistant to the allure to chaos.
Very true, but are the Tau capable of detecing Chaos taint in the same manner as say the Imperium and the Eldar are? In the case of resistance I would say that its more to do with the daemons being uninterested in the Tau than the Tau having a resistance on their own. In practice it should however be fairly close.
And do they really understand the difference between aggressions of the followers of Chaos and say the Imperium? Because without and understanding the two of them might seem fairly close in a general love for war/total intolerance and wish to victimize others.
This does not mean the Tau are gullible to become slaves to random space marines. The Tau may be a little naive concerning galactic scale, but they are also fairly self-centered. They don't want to co-exist and work with everyone, they want everyone to exist together and work under them. They aren't going to simply subordinate themselves to a random dude who comes around and saying, "look at me, you should really do what I say."
No, of course they won't bow down to the Marines. The idea I'm thinking about is that a bunch of smooth talking Marines comes along to offer themselves and tell the Tau about a certain knowlege regarding Warp travel that can be found, and that the Tau lets a few of these Marines be part in an operation to find this info and bring it back to the Tau as the Tau might consider this option to have both Space Marines working for them and the possibility of getting a bit further on the road to develop Warp-engines to be worth taking the Marines on their offer, or a least a few of them.
Not all the Marines will be able to get out again but their leader might bull them to get along with the plan and only really care about getting some influence over the Tau so nudge them a little in his desired direction - outright control will of course be out of the question.
If the space marines try to infiltrate the Tau the Tau will be really smart and ruthless with them. They would accept them sure, but on their terms. The warband would likely be split up with each member put under the command (read: watchful eye) of a Tau leader. They would be denied their autonomy and allowed to act only when the Tau wish them to. Their technology (power armor, power weapons, bolters, etc.) would be confiscated for study with "replacements" given until such time their gear has been "improved" (bugged, fail-safed, etc.) and returned.
That seems reasonable and something that the Marines could work around as it would give the Tau as sense of control over the Marines which could lead to them allowing the Marines to spread both the corruption as well as making the Tau listen to their suggestions. Do you think that the Tau would keep the Marines closee enough to allow the Marines to cultivate influence, and perhaps spread their corruption, to Tau leaders?
They do the same with all Guel'la, sterilizing humans who turn to them and making them second class citizens. The Tau really aren't that nice.
I agree entirely. The Tau gets way to much goodie-goodie PR to do them justice.