Corrected entry: If an alien creature comes from an implanted human, there would only be as many creatures as humans, but there are far too many aliens. Remember the tunnels with the automatic machine guns? Those guns, supposedly, killed hundreds of aliens.
Correction: The aliens were bred from the implanted inhabitants of the colony. There were plenty of them to supply the amount of aliens seen.
Correction: There's a lot of overkill from the sentry guns as they simply fire whenever they detect movement and are unable to account for size, etc. Note that the guns fire multiple rounds just to take out the single empty canister thrown in front of them for a test. So the sentry guns having fired hundreds of rounds doesn't mean they've taken out hundreds of aliens.
Corrected entry: After they escape the planet, the nuke goes off, and they are back on the spaceship, where they should all be weightless, including the alien, and would be floating around on the ship, not walking around like they were on solid ground. This totally unscientific approach to the movie is annoying.
Correction: Well, you must just hate science fiction, then. Practically every sci-fi film set in space features a ship or station with artificial gravity. This is a fictional technology, as are faster-than-light drives, hibernation pods and androids made in human form, all of which feature in the Alien series of films, all of which are unscientific and all of which presumably annoy you as well. Unfortunately for your sense of scientific indignation, the use of a fictional technology in a science fiction film is not only not a mistake, it's practically a requirement.
Correction: They are not weightless when they wake up from cryosleep at the start of the movie either. The ship obviously has some form of artificial gravity.
Correction: In addition, it's emphasized multiple times that the xenos are smarter than expected (example: the "They cut the power!" "What do you mean THEY cut the power? They're animals!" scene). It's highly possible the xenos only lost a handful of their number to the sentry turrets before working out a means to safely trigger them until they ran dry (TV Tropes proposed the idea of them hiding behind cover and waving their tails to set off the turrets' motion detectors).