The idea that Tau are incorruptible is crap. The warp can drive anyone (Pysker or not) completely insane and once you are it becomes very easy for you to become a pawn. Also the idea that all Tau are too stalwart or focused to succumb to overwhelming temptation is again crap. If Chaos can corrupt a Primarch they can do it Shas'el.
lets not get to wrapped up in tournaments and canon. The fact is that Tau are as corruptable as any other race, no matter how focused they are in the greater good. Insanity is insanity people! As for the amry itself, I suggest having it be a chaos army with tau models as "count-as" models. With some conversion work, It would be a snap. eg: Fire warriors=CSM, Deamon Prince= Shas-O to name a few.