Well, if Chaos wants to deal with the Tau, now they have Culture backing, there's one obvious route that the sneakier elements of Chaos could should and probably would take.

Give the Tau rope. Put them in situations where they are forced by their own beliefs, morals and failings to do horrible things, such that the Culture will be forced to deal with them, turning the Culture from a problem to a solution.

Nudging the Tau in such a way that they become increasingly genocidal towards alien(or at least Human) life, only with the Culture-backed prowess to do something about it would be the obvious way. If the Tau suddenly start exterminatusing every human planet they can find using the advantages they have been given by the Culture, then I can't imagine the Culture will be able to just shrug its shoulders and write the situation off as a minor dissapointment.