I have the feeling that a Tau army always depends on much synergy between its units. Every addition of a non-Tau unit seems to be crippling, since it doesn't work together with the rest. That being said, Tau lack close combat defense. Once the enemy reaches you, you are screwed. I can imagine Wraiths and a blob of warriors as nice working tarpits to defend the gunline.
I don't really know what
CSM could offer tau. Their suits are better than our vehicles and in an allied detachment you can't take
PMs as troops, which would be a good addition due to durability, which Tau lack. The same for
MoN Bikers. Obviously renegade and Chaos knights aren't bad. Sorcerers for psychic defense would be nice. So... 1group cultists, 1 sorcerer with Bike, 1 group Bikers with
MoN, 1 group
PMs with Rhino to take Objectives. But again, you probably can take some Tau units for those points with more synergy for the rest of your army