...Which may be kept for Mark o t Xeno's.
What they should have done is a have Tau collective alien species that is based in the region and fights in their armies locally, kind of like the Vespid (which were first seen in DoW btw).
It would have been good for the rp'ing (if that matters) and they could have created it for a specifc challenge, bearing in mind that the protagonists are going to be almost exclusively Space Marines.
Of course, being anti marine is kind of what Vespid are for, but you can be a lot more subtle in this scale. I don't have any ideas atm but perhaps someone else does.