couldn't Weyland Corp have rented out the marines or something. And picture it like a super mega comanay that owns and runs almost everything.
You don't "rent out" Marines, no.
The closest you get to that, in today's world, would be private security contractors, like Blackwater. If a government ever allowed private companies to briefly own their own defence forces, it would be insanity.
And like I said, doing so would contradict Ripley's conversation with Burke in the APC about company jurisdiction versus military jurisdiction, which was more realistic and also served to make the stakes more interesting by introducing the concept of competing interests.