Sort of my point 'Splodge - a real world caste system is nasty because someone has to be on the bottom. I don't think that necessarily applies to the Tau - the Earth Caste would be the natural fit for the peons of Tau society but the fluff has them and the Fire caste as the main protagonists of the civil war. There is fluff of Sept worlds where the Earth caste are in ascendance.
Actually I think the problem is trying to apply real world psychology/sociology to a fictional setting - I don't think GW intended Tau to be evil and illustrated it with a caste system - because I expect whoever wrote it wasn't aware of the inherent real life ills of a caste system.
Also, the 'join us or die' aspect creeping in to the fluff is probably a side effect of 'make 'em more grimdark' rather than 'make 'em more evil'. No one wants to have an army of peace loving hippy flower arrangers after all. Destroying all opposition with mecha is just what tyranids do with big gribblies. And if the writers can claim the intent is for the greater good, so much the better.
As to fish people, I always thought that was the case - probably wrongly - never really gone for Tau.