Xenomorphs have some sort of non-verbal communication of a hitherto unknown format. It could be psychic, it could be EXTREMELY AWESOME pheromones, it could be Phlebotinum. So. That question doesn't have a set answer. Let's assume both yes and no.
Yes, then the Xenomorphs know the Thing has assimilated one of them, and they immediately try to kill Jeff in hand to hand. Which fails utterly, because he's the Thing. Anytime you wound him with melee, you're going to cover yourself with Thingcells. And if they spray acid on him, whilst he has assimilated an Alien, then it's going to be ineffective. I guess they could non-lethally overpower him and pin him to the wall with Alienwallstuff, but then he'd Thing his way off by transforming into a worm or something.
No, then the Xenomorphs get overpowered one by one, or worse yet, the Queen gets Thingified... And shit gets very, very bad for everyone else in the universe.
The Aliens are smarter than that. Much smarter. They would keep their queen the hell away from the infected, and direct the humans towards it. Because humans are better at burninating aliens. Murphies have demonstrated they are aptly capable of manipulating violent humans, with one particular gifted Murphy outsmarting an entire spaceship full of SPEHSS MEHRENS. (Spehssmericans though, no massive pauldrons ;( ).
Wait.
Void Dragon awakes, gets hold of Death Star.
Actually an Imperium x Empire crossover where the void dragon awakes would be a beautiful thing. Why?
Every single piece of technology the Imperium uses that isn't Xenos or low tech would be under its control.
The Empire doesn't use imperial technology.
Suddenly these weird psyker human things are the only hope left of filling the gap the Imperial Navy left.
Sadly I do not know enough Star Wars canon to write it myself...
Void Dragon is canon. What exactly it is and what it is capable of are not canon. The Emprah meets the Void Dragon in the Mechanicum Heresy book, and seals it away beneath Mars, out of reach of the Techpriests.
And this is after the Void Dragon had just fought Vaul, the Eldar GOD of technology - with an army at his back, and got blasted in the face with two giant psychic laser things that are designed to eat up entire star systems.
And the only effect that had was it made the Void Dragon sleepy.
The Empy fought it while it was sleepy.
What's to stop a human-biomass sized Thing from simply outsmarting the Alien? Aliens are quite intelligent, probably as smart as a particularly intelligent dog, but a Thing in manform is as smart as a man. Hell, one made a SPACESHIP IN A CAVE WITH A BOX OF SCRAPS WITH A BOX OF SCRAPS.
Murphies are collectively intelligent, with individual Murphies often showing unique intelligence capabilities. On the other hand, the remake movie of Jeffrey shows a similiar hive mind for Jeffrey; where it didn't rely on the host's intelligence but rather had its own innate intelligence, being capable of building its own ship instead of accidentally landing in the middle of fucking nowhere.
Murphies are considerably smarter than humans. We don't know the intellectual limits of Jeffrey. This is canon.
With humans in the equation; Murphy wins. If it was a simple 1 vs 1 of Murphy vs Jeffrey, Jeffrey wins.
Murphy wouldn't fight Jeffrey directly, ever.
Murphy's adaptability limits far exceeds Jeffrey's given enough time. Jeffrey can change itself rapidly to fit the situation; but not to the limits of Murphy.
As I said in the previous derail, Murphy has been found adapted to fit all kinds of roles; taking a dive in molten metal in a moulding press, living in the cold vacuum of spehss, adapting to be compatible with human reproductive systems - there is no end to what they can do.
They change features like people change tools.
Also, we should probably assume that all 'special abilities' from all the universes work on each other. The Thing can assimilate the Alien, the Alien can facehug the Tau, the Psykers can mindcontrol the Thing, etc. Otherwise people just get all pissed off and shut down. Oh, and the Thing could probably take over at least part of the Necrons. The Pariahs are part human.
THE PARIAHS GOT RETCONNED OUT BY MATT WARD
FUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU
Why are there no Inception Matrix crossovers?
Think about it! A dream inside a dream except that it was all just the matrix except was the awakening a dream, maybe the matrix was just limbo and you've been in the battery world this entire time but what if that is an idea planted inside your head?!
Too sexy to exist. Neo telling Cobb there is no waifu. Mr. Saito telling Morpheus he's wearing a leather coat within a leather coat.