Here's the biggest problem
imo:
Comparing the Tau to the Covenant isn't really a good exercise, just because we don't know the full picture of either/or. We KNOW that each is several different "castes" ruled over by another, but the exception being that the Tau follow the Ethereals fairly blindly, while the Covenant DID start questioning the Hierarchs and broke into two separate factions(the Loyalists--the Hierarchs, Brutes, Drones, a few of the Grunt clans it seemed and the Rebels--Elites, Grunts, Jackals, and the Hunter tribes).
I'd, personally, say though that the Tau would adapt after the first few engagements. The Imperium utterly crushed them in the opening operations of the Damocles Crusade, and then were(up until withdrawing to counter a Hive Fleet) being beaten back, hard. On the ground, the Tau would have a *huge* advantage in that the Covenant rely very heavily on waves of troops and masses of warmachines alongside of their caste system specifically for command(Brutes leading the Jackals, Jackals leading the Grunts, etc), while the Tau rely on heavily trained troops, squad leaders relating back to a trained military commander(admittedly, who DOES report to a religious leader much like the Covenant), unit cohesion, and combined arms tactics.
The way I envision it going? At least on an open/built-up area that the Tau had time to prepare.
Infiltrated Pathfinder and Stealth teams draw the Covenant into the set trap, the Fire Warriors open up and draw the Covenant further in, and the Pathfinder/Stealth teams go to work on the leaders breaking the back of the Covenant C&C. Or in the case of a Covenant Armoured Assault, Pathfinders+Stealth teams would light them up for Skyray barrages and the inherent range on the Hammerheads/Broadsides, the mobility of the Stealthsuits/Piranhas/Crisis Suits would come VERY heavily into play.
Now, a free for all in close quarters like a boarding action? I suspect the Covenant would do much, much better.
But, I'd root for the Imperial Guard against both