A plan that deals with the Tau without allying them? Yes please! This plan only has one flaw though, don't forget to invade the Tau while they are fighting. Don't get too close as to enter a battle of Tau and Chaos but close enough to threaten the capital. The Tau will probably be worried and offer peace. However get a couple of Space Marine companies waiting to invade the weakened capital if the Tau disagree to Imperial demands. Make those demands so tough that they will weaken the tau but lenient enough to make them agree. Wait till the peace treaty takes effect then invade anyway. You will face a weakened Tau force, weakened both by the war against Chaos and the demands of the treaty. In this way we exploit the ability to make peace with the Tau and then crush them!
Misha, you sure you are not of the Alpha Legion?
Serpentis Fidelis