So, the Tau traits presented were to represent Tau who were still a part of the Greater Good - useful for all-Tau games, but not great for mixing with regular humans. So, here's something that might help...
Note: This Trait replaces the For The Greater Good Trait possessed by other Tau
Not all Tau follow the Greater Good. Those who have found themselves seperated from the Ethereal Caste (most famously, Shas'O Vior'la Shovah Kais Mont'yr, more commonly known as Commander Farsight) find their opinion of the Greater Good waning, and some actively oppose the aggresive expansion and extremism of their own people.
Renegade Tau lose the For The Greater Good Trait, and the associated benefits and penalties. Instead, they gain a -10 penalty to all interations with non-Renegade Tau, but are much more free to interact with other races - gaining no penalties to interactions with them, once their renagade nature is known. (So, a human might have a bad reaction to any Tau, but once they have been seen to reject The Greater Good, and swear their service to The Imperium, they are no longer seen as such a threat).
They may also purchase the Mercenary Trait at character creation, for 100xp. This functions as the Kroot Trait (the character may use the group's Profit Factor instead of their own Willpower to resist Pinning and Fear).
When in the presence of an Ethereal, Renegade Tau must make a Difficult (-10) Willpower test, or succumb to the will of the Ethereal (losing this Trait, and regaining For The Greater Good.)
At the GM's disrection, non-Renegade Tau who have been removed from the Ethereal Caste, and other non-Renegade Tau, may make a Difficult (-10) Willpower Test each day (or at dramatically appropriate points) to resist turning from the Greater Good. Success means they remain loyal, but failure means the lose the For The Greater Good Trait and replace it with the Renegade Trait.
Tau who have turned Renegade after character creation may also buy the Mercenary Trait, but as an Elite Advance, costing 500xp.