Due to problems with the quote function (*shakes fist in anger*) I'll be using Italics to indicate the statements I'm responding to and regular type for my own work.
The Tau don't have the numbers... yet.
Best case scenario is that they won't have anything like the numbers for a long, long time. They have a couple dozen planets with a total population less than that of Necromundia. The Imperium has more than 30,000 hive worlds.
And they aren't going to grow fast. They're under threat and attack by Orks and Tyranids and that's without even bringing humans into the issue.
Their technology is constantly leaping forward, in a millenium they will probably equal, if not surpass, that of the Dark Age of Technology .
That's an optimistic interpretation. Here's another one. Their technological progression plateaus and stagnates. That's optimistic, because it assumes they don't get eaten by Tyranids.
Their empire expands rapidly, at the moment the only thing really stopping it is the Imperium. Once the Imperium shatters, the Tau will be able to absorb human worlds with ease. Again, their culture was designed with this in mind - their willingness to accept non-Tau into their empire allows them to expand quickly, especially with the amount of human worlds out there that will be just waiting to be gobbled up.
No, it won't. They local humans will still be there and a few densely populated planets have more industrial might than the entire Tau Empire. The Tau don't have the population of a single subsector and given the feudal nature of the Imperium, organization will survive on those low levels on many areas. Given the Tau's crummy FTL and lack of astropaths, their empire will have far greater difficulty with large scale organization than Imperial remenants.
And that's ignoring the Orks and the Tyranids and the Necrons and everyone else. Or Ultramar deciding "these guys need to die."
This will, as I said before, take a long time. And it's true that another force could wipe them out before they succeed. But it is quite definately a possibility. That is the horror of the Tau - they are the successor race for humanity. They have been designed with the fall of the human race in mind. Someone out there (the Eldar) have forseen the end of the Imperium and coldly decided that they needed an alternative race to impose order on the galaxy.
Basically, the Tau plot is clearly taken from Asimov's Foundation series, which is pretty much exactly the same accept all the factions are human.
40K steals from everywhere. And 40K's plot owes a lot more to Dune than Foundation.
The Tau were intended for something alright. Whether or not that will come to pass is another thing. Their numbers and position (right by Ultramar of all the places in the galaxy) make it unlikely that whatever Eldar faction created them means to use them to "take control of the galaxy", something the Eldar view as their rightful spot anyway. Anti-chaos, anti-human cannon fodder armies to bolster the Eldar's own lack of numbers makes much more sense. It'll bolster the eldar numbers, being a blunt race their resistant to possession, they are conveniently controllable through the ethereals, and could be a potent force when directed to fight and die at the right spot by farseers.
As for Tyranids and Necrons, it'll be the Necrons. The 'Nids aren't interested in unliving matter and much of their biotech and control mechanisms is based on warp power. Once the Necrons complete the Great Ward and seal the galaxy off from the Warp, the Tyranids are toast.
However, there's hints that the Tyranids are just servants of a greater and more terrible master.
