Umm…there seem to be a lot of misconceptions about the Thing. It mimics it’s victims on a cellular level, for one thing. So, if one is the size of an alien, and has at any point gotten a sample of it’s DNA, it can create an exact copy of it. As a dog, it’s difference was completely unnoticeable to the other dogs in the shed until it started to hiss and mutate. I’m guessing that if dogs can’t tell it’s a fake, then aliens wouldn’t be able to tell the difference between a Thing copycat and an original, seeing as how they most likely base all their observations on smell as well, which the Thing would be able to duplicate exactly.
As for Zen 101’s observation, the beings in the movie and the game have fluids in them because they duplicated creatures with a circulatory system. If you note, according to the movie (and the book it was based off of), each blood cell is an individual being for the most part. It’s simply been designated/chosen to take up that particular form to help the creature be more like whatever it is it’s copying. If enough blood is removed, and the creature feels it’s in danger, a pint of blood can turn into a snake or small animal and get away. That new animal would have a completely new circulatory system, complete with blood and other fluids (venom, lymph, bile, etc…).
Also, the thing can infect a creature and take it over from the inside out in time. If the Thing tosses spores or something on an alien, it can assimilate the skin, then move into the veins, and duplicate every aspect of the creature necessary to survive with the caustic blood. Now, it’s ability to duplicate the blood, that could be questionable, but even if it couldn’t, it could simply stick to controling the circulatory system itself, and the alien would never know it was infected and being taken over until it was too late.
Due to it’s perfect duplicating abilities, I’m pretty sure it would also seem completely normal to most scans done by any of the predators’ equipment, especially if it’s simply checking body heat.
Overall, the Thing is the ultimate alien. Even if it’s blasted to bits, if those bits aren’t electrocuted or burnt to a crips, they’ll either reform, or go their separate ways and find food elsewhere, which makes it even more deadly.
But I think the Blob could handle all of them.