Inquisitors all have their own independant plans, ideas, experiments, modes of thought and so on. They have the freedom and resources available to pursue these lines of investigation. Some will inevitably focus on the Tau. They are a threat to the Imperium, just as Orks and tyranids are.
First, they represent a threat to the Manifest Destiny of humanity to rule the entire universe. Monodominant Puritan Inquisitors would like nothing more than to burn every single xeno creature in the universe, from the smallest slime to the largest tyranid, on a might pyre before the Golden Throne. Most inquisitors are a bit more moderate, and are happy to leave the safer species like Jokero to their own devices, but the Tau... yeah, that isn't an option.
The Tau are an aggressive, expansionistic empire deliberately targeting Imperial worlds on the Eastern Fringe with armies and propaganda. They engage in the foulest heresies, from technological ones like the embracing of AI, to the moral ones like attempting to unite all sorts of other filthy xeno under one banner. Their active research programs present a moral threat to the orthodox Adeptus Mechanicus, especially when they break apart Imperial tech to see how it works, while also raising the horrible spectre of a return of the Iron Men. They accept traitors to the Imperium into the ranks, which is bad from a law-and-order perspective, to say nothing of potential sources of corruption. They don't listen to the Imperiium when they try and warn them about some of the mistakes they are making (poking Necrons is a really bad idea, Space Hulks have bad reputations for a reason, there are really good reason for prayers and chanting around a Chaos artefact...). When the Imperium went to war with them, the result was a stalemate. They are a tremendously young race that within