By the background, Tau barely register in the warp so are not terribly easy targets compared to humans. Also the Tau aesthetic and modus operandi don't fit well with the MOAR SPIKES MUST PUNCH IN FACE route that chaos armies tend to go down
So if possession is unlikely you need to ask yourself if the Tau are actually part of the cult/warband, or if they are simply normal Tau with a common goal or have been duped into assisting without kbowing what actually drives the chaos force.
Or you could whomp up some possessed battlesuits with gribbly bits and tentacles in a similar way to the hellbrute.