Cool to have, but hard to tame. Nexus, if I read the Star Wars site correctly, are extremely difficult to domesticate. Xenomorphs are...well...I'm sure anybody familiar with the "Alien" movies knows what xenomorphs are like.
But both of them, combined? Fox had to have had some sort of containment unit, and a lot of patience. Though, while difficult, I imagine it is possible. Nexus are, indeed, often used as guard...cats. Though..could you imagine what that thing would do to an intruder?