According to an interview that the film's director (Neil Blomkamp) did for Io9.com, there are definitely other life-forms out there, aside than the Prawns and the humans.
The other thing is that the ship was meant to clip together with other
ships. So there's, like, vast amounts of resources that they're
bringing to the parent planet. And the ship, when the army generals or
the queen of that particular ship died off by some sort of virus or
bacteria that they picked up on some other planet, that killed them
And it didn't effect these sort of resilient, hardy sort of drone
workers. Then the technology is usually the thing that they relied on
to save them, but in this case it sort of screwed them because it
brought them to a planet that kind of treated them pretty badly, but
it was the ship that realized that, unless it gets to a life
sustaining planet everything is going to die, which is a cool idea. So
the ship just autopilots to the closest one in the Goldilocks band,
and it's our planet and then pulls up and hits the brakes.
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Obviously bacteria don't need to be kept at bay with guns and mech-suits, but the simple fact that single-celled organisms can be found in our galaxy would strongly imply that there are other higher lifeforms as well.
As regards the possibility that the weapons are for use against their own kind, that seems extremely unlikely.
The hive mind [concept] is the most important thing to me, because I love the idea of a civilization that can build all of that technology and then, at the same time, just have a massive population that was just drones that needed direction, and were absolutely incapable of building that stuff on their own. I found that to be a really interesting concept. Also, it sort of explains why they don't turn on the humans. Individually, they may be feeling oppressed, but they don't have it together enough to form a resistance and back one another. So I found that really interesting.