Unless the Supplement has a special matrix (I haven't read it), I would say they use the regular one. Which sucks since there really needs to be some way of doing Gue'vesa
I have a copy of the original Gue'vesa rules. Which likely could be used in friendly games. For official tournament, or play in a GW store, you're likely out of luck.
To my knowledge, unless the Supplement book containments a spefici matrix for Farsight Enclaves, it would likely follow the same Allies matric as per the rulebook, so iirc, that means Tau can ally with Imperial Guard, but there is a chance that units from the two detachments will stop what they're doing to stare violently at eachother.
Don't quote me as a rules lawyer though