1: The Tau are perfectly content with expending their troops lives when necessary to advance the Greater Good. Thousands of Fire Warriors might fall in the invasion of a planet, and that is just fine. Don't mistake tactics for strategy; the Tau abandon defensive positions when holding them would cost their lives with no hope of success, or when doing so allows them to force the enemy to overextend so they can start hit-and-run attacks and cut the enemy's supply line. It's all part of the 'well-managed' bit. They avoid tactics which would cause unnecessary casualties, but are very willing to accept 'necessary' casualties. Unlike the IoM, they recognize that you can't conquer the galaxy if all your soldiers died defending a worthless bombed-out bit of trench.
2: Squark already brought this up, but as long as you weren't born on an Agri-world or a Forge World, you've got a decently large say in what sort of job you'll have.
1: No, the Kroot are one of the first members of the Tau Empire. They think of themselves as part of the Tau Empire, and for the most part remain loyal to it. They send out Mercenary Kindreds specifically for an opportunity to meet new, strong beings and eat them to advance the Kroot species. They keep this secret from the Tau because they know the Tau wouldn't be pleased, but the Kroot homeworld is within the Tau Empire and the Kroot are officially one of its client species. Yeah, the Kroot aren't mercenaries for the Tau to the point that they go to huge lengths to keep the fact that they do any mercenary work at all secret from them. Note that this was one change from the 'old' Tau codex to the 'new' one. In the 3rd edition book, the Kroot were just mercenaries allied with the Tau.
2: We don't know much about the circumstances of Farsight's rebellion other than that the Etherials on the mission died about the time he rebelled, and it probably has something to do with that fancy sword of his, or at least where he got it from. Farsight and his pals are one of the official mysteries of 40k, left intentionally vague so players can fill in the blanks themselves. It might be that he couldn't have rebelled without them being dead. It also might be that he killed them himself. Either can be read from what little we've been given on Farsight's rebellion. Heck, one theory I've read states that he's still working for the Etherials, acting as a beacon for malcontent Fire Warriors to form a wall between the Empire and something nasty on the other side of his Enclaves.