A lot of claims are being made about the Thing's abilities as if they are irrefutable facts, but they are not. I don't believe a Thing can assimilate an enemy by merely touching them; All of the assimilations seen in the movie were quite violent, involving more than just touching. It was not verified that a single cell could infect an entire organism, it was only speculated by the character Fuchs, who seemed to be out of his depth.
THE THING was a great movie, but it is clear to me that the how/when/why logic of the way the creature worked was not thought through very much. The Thing's abilities are open to endless, endless debate.
It's perfectly plausible that a Thing could infect an Alien. It is equally plausible that the Alien's acidic body chemistry could make it immune to assimilation.
With that said, I would find an Alien/Thing match-up interesting only if the Alien were immune, and not just because I'm a bigger ALIEN fan. I say this because if the Thing could take over an Alien, then it really would be no contest. There could be some cool fighting, but the result would be a foregone conclusion. Who wants that? With the infection factor eliminated, we could concentrate on how badass the battles could be. Imagine what kind of creatures the Thing would imitate in order to fight the Alien, and how the Alien would react to each new incarnation. That would rock!
As for the Predators, they would do at least as well against the Thing (probably much better) as humans could do. One of their vision modes could probably detect the Thing, or maybe they couldn't. Either possibility is legit.