Well, it is not unprecedented for the Tau to find a way to control another race against their will. This is strongly hinted at in the entry about Vesdid, and the fact that they refused to ally with the Tau until they were presented with the Communication Helms. Once they put those helmets on, the entire race immediately ceased resistance and bowed to the Tau. This would be next to impossible to do with nids however, as the nids are not sentient, and their only emotion is hunger.
Another thing, the Ethereals control over other Tau is on two levels, and neither has anything to do with psychics. The first and most basic is a pheromone that they are able to release from an organ in their forehead. Upon scenting this pheromone, any other Tau loses the ability to oppose said Ethereal. The second and more recent is deeply psychological. From birth, they are trained to obey the Ethereals above all and without question to such a degree that it takes an intense emotional strain to overcome it.
Neither of these control measure would have any effect on the Tyranids, and so extermination is the only answer.