Kroot have a minimal instance of Chaos corruption and minimal psychic talent. While they aren't dim souls like the Tau, they also have no role in the actual running of the Tau empire and would be bad targets for divination efforts. Corrupting them is an awful proposition, because their culture is very opposed to the unpredictable nature of Chaos thanks to their focus on self-driven evolution and personal improvement. They can smell and taste Chaos taint, and they don't like it. The Vespid are a socially-oriented hive mind with minimal concept of individuality and might (maybe) be outright mind-controlled by the Tau. Chaos would have about as much luck corrupting the Tyranids. Demiurge are allied to the Empire, but have little direct investment in Tau society and would thus be a bad vector. Nicassar are very, very tranquil minds and are by far the most stable psykers around (you would be too if you needed to push your starships around with your mind for your entire life). Harder to corrupt than Eldar. Ji'atrix and Galgs are non-canon concept pieces, unless they've been mentioned again in a recent source I haven't read.
The only current corruption vector in Tau society that would make any difference at all are the Gue'vesa. Every other race is either very resistant to Chaos taint or so tangenital to the Empire's rule that they won't make any difference to begin with. The Sorceror is going to have a real uphill battle manipulating the Empire; the Etherials hold all the cards, and they're virtually impossible to corrupt. It may just be too late to take the Tau off the playing field; the best means would have been to wipe them out by force before they got involved with the Culture, but that may not be possible with Culture protection. Otherwise, his best bet is to destabilize the Empire with human agents, encouraging the Tau to crack down on their human populations and maybe alienate the Culture to some degree. It's not a total solution, but it might just mess things up enough to give him room to work on a more effective vector of targetting the Culture.