Yeah, Tau carapace armour seems to be your worst enemy right now. I've started writing up part one of the raid, should be up in an hour or so.
Also, more for my own amusement than anything else, here's a quick rundown of my headcanon regarding the Tau's attitude towards enemies.
Fire Caste tends to be fairly aggressive, especially in lower ranks. They're also short and to the point, leaving the diplomacy for the water caste. Higher ranks vary a bit; some make their own code of honour and enjoy warfare for it's honour and challenge, some only get more aggressive and xenophobic (Brightsword). Most follow the standard Tau doctrine of living for the furtherance of the Greater Good. They're usually pretty professional, don't have much ego and won't waste time on things they view as lost causes (like interrogating McTavish)
Water Caste on the other hand tend to appear very friendly, but they're actually often almost sociopathic. Trained from infancy in the art of linguistics, if you talk to a water caste member then they will leave the discussion with more information than you. They are trained liars, often skilled in psychology, and very thorough in fact checking (Por'vre checked every one of McTavish's aliases, including Guilliman). Standing orders for the guard state that captured water caste agents are to be transferred to high command immediately, or executed if that isn't possible. They just aren't worth interrogating most of the time, because they screw you over nearly every time.