tau usually wouldnt try to infiltrate deep into imperial territory, as they have trouble hiding. but they have no trouble hiring and getting humans to turn to their cause. tau is also most likely to help out indirectly by creating xeno-technology that traders would be very interested in getting their hands on. certain seeds and plants genetically enhanced to create food or other rare substances that are popular in worlds where imperial law and religion isnt the strongest powers around.
Whered u get the stuff for the second Necron thing...might see use...(o o)
Tau are certainly one of the more challenging races to use as bad guys because they are not black and white evil like the Tyranid or Chaos. These guys have motivations and emotive responses. They are psychologically more similar to humans than the eldar are (IMO) and this makes them rewarding villains (or allies?).
concentration, labour, and extermination camps (perhaps in the same complex)