engineered bioweapons'. They simply have a totally different instinctive strategy. We apparently don't have a problem accepting that Orks are, next to humans, intrinsically savage and crude 'because that's just how they are', but the idea that another species in the same setting could be, next to humans, benign and tolerant is, for some mysterious* reason, met with resistance.
No, the tau are not just biological robots. But given that morale impacts performance, it strikes me as pretty counterproductive to engineer a sub-species with talent for a particular set of tasks, without the enthusiasm for applying said talent. I suspect that the various castes have, with very few exceptions, corresponding proclivities for their broad roles in life. It's conceivable that the Tau deliberately thwart the ambitions of a minority of oddballs, but it's also conceivable they try to accommodate 'strange tastes' where possible. We just don't know.
* Not actually mysterious.Not much has been said about the castes interbreeding, but I get the impression they can't anymore, or at least if they can, there are no half-castes.
I imagine the castes do socialise, but you need to remember that these are gene-jacked aliens created for a purpose by even more enigmatic aliens, and there's only so much use to human psychology.
While they mostly act like Ghengis Khan's golden horde, as equipped by a 23rd century Leonardo Da Vinci, they aren't humans. All we know for sure about their society's break points is that constant expansion is needed to keep the fire caste happy, and that taking away the Ethereals is a bad idea. Oh, and that it works.