The Tau don't apply the Caste system to other races at all. They aren't Tau, after all. As far as I can tell, the Tau think of other species as sort of castes unto themselves. Kroot get to be guerilla warriors, scouts and woodsmen (all jobs that they relish), Vespid do... Whatever the hell Vespid do when they aren't fighting with the Fire Caste? I mean, this is one obvious failing of the Tau fluff, because we honestly have no idea. Nicassar get to hitch rides with Tau ships in exchange for warning the Empire when 'Nids are about to show up, Demiurge are basically left alone to mine and trade however they want to, Gue'vesa are mostly left to their own devices with minimal interference from the Tau. There are no Gue'por or Gue'shas, because humans are all pretty much genetically the same.

Again, and as Carry2 has been pointing out, the Castes are biologically and socially distinct. It's not eugenics, it's obscenely high-speed natural selection (though the Tau do practice breeding programs within the Castes). The Fire Caste spent most of their time fighting, so they got big and tough and warlike. The Air Caste lived way up in mountains, so they developed a lot of birdlike features so they could glide around. If you asked a Fire Warrior whether he resented not having a choice of career (and they do have some choice, to some degree, despite being the most constrained Caste by far. Every other Caste has many times the job diversity the Shas do), he'd ask you whether you resented not being born with wings.

Also, anyone who thinks the Imperium is more free than the Tau is just wrong. The motto of the Imperium is "Know Your Place". You can't dream of becoming a planetary governor because you aren't a Noble. Too bad. You can't dream of joining the Mechanicus because you didn't get initiated when you were three. Too bad. Nope, the Administratum has no intention of hiring an illiterate bumpkin. I made this same comment earlier, but unless you're lucky enough to live in an area where Imperial control doesn't really extend to, like an Underhive, you have no freedom other than the choice whether to join the Guard. And if not enough people volunteer, they take that choice away from you too. If you're supremely unhappy at whatever you're doing in the Tau Empire, they'll find a new job for you. Because unhappy workers are unproductive workers, and they want you to be productive. In the Imperium, they'll flog you for not working hard enough, then tell you to do your job For the Emperor.