This is part of an e-mail I sent to Randy Pitchford of Gearbox Software, essentially asking
him of what was Race X's role in the Half-Life universe. I would like to thank him for
taking the time to very kindly respond to my rather insignificant questions. :)

(Me)
:(Q) 1. In Opposing Force, are Race X and Xen creatures (the Nihilanth's forces and such) intended
:to be enemies against each other? Or are they allies working on the same side? They are never
:seen fighting ingame (or together in the same area, for that matter), but some believe that
:there's evidence to support them being hostile to one another, and vice versa. 

(Randy Pitchford)
::(A) 1 – I worked within a framework articulated to me by Marc Laidlaw of Valve that allowed
::for many alien species to exist in the universe – some intelligent, some not – some with
::agendas, some without. With what was revealed, you can deduce that the masters of the aliens 
::referred to as “Race X” clearly have some sentience and some agenda and they clearly employ or
::subjugate other aliens…   In that way, they are similar to the creatures that have subjugated
::the Vortigaunts (and you learn more about that in HL2).    Other than that, there is no clear
::answer to your question given the information that’s been revealed by the games.