At the risk of sounding a tad blunt, I'm guessing Crystal Maze is asking about Tau because that's what he wants.
From a contact's point of view, it's probably a lot easier if they're dealing only with one species, rather than multiple types of aliens or Gue'vesa, unless it's the Gue'vesa that's the first point of contact rather than the Tau itself. On the other hand, I imagine it puts the Tau at a disadvantage if they have to introduce the non-Tau as "a subject of the Tau Empire" and risk creating the impression that the Empire consists of "the Tau, and then everyone else".
It's probably easier to limit the number of species in the group to avoid making it look like a cast of token minorities, whereas having multiple Tau creates a stronger impression that it's a Tau warband and that the Por caste guy should be the main focus.