I know that Marines are elite troops and that this great film mimics the failings of the Vietnam War, but wouldn't a real offworld danger involving dangerous alien organisms need a far larger body of soldiers?
And there in lies your answer...
You have to realise that Mr Cameron, in his intent make a Vietnam style movie, used his sequel to that end, and in doing so pretty much ignored most of the facts about the Xenomorph we found out about in the original..
Why were they terraforming LV426 anyway? In the original movie it was made clear that Weyland-Yutani already knew what was on that moon by Ash's 'Crew Expendable' massage, in this movie it seems that they think that they had simply lost contact.. Ripley convinced them it was more than that.
Where did the queen come from? In introducing the Queen Cameron pretty much destroyed the Alien, turning it from 'the perfect' organism into giant space insect... While it was virtually 'unstoppable' in the original to being killed with a shotgunin this sequel.
And introducing the Queen, Cameron also destroyed the unique life cycle of the Alien, From 'egg', to facehugger to chestburster to adult. originally the Alien propagated itself by selecting not to use all of it's victims as food, and instead injecting a few with an enzyme that metamorphosises the unfortunate victim into an Ovomorph (egg), no Queen required.
And, as the alien can only produce one alien using one victim, how come there were so many Xenomorphs on LV426 anyhow? There were only a limited number terraformers to use, of which most were still alive when Ripley and co. arrived.
As for why they were terraforming lv426, remember that the events of Alien were 50 years in the past at that point. Everyone from W-Y who knew anything about it was probably dead from old age, or at least long-since retired. There might have been some secret files hidden away somewhere, but it's also very possible that 50 years ago they just covered it up, destroyed the evidence, and never talked about it again.
That is very true, and the storyline of Aliens does support that, as it seems to be Ripley's testimony into the destruction of the Nostromo that caused W-Y to send the colonists to investigate the derelict ship.
Though I still find it strange that, with all the possible worlds out there to try and terraform, W-Y chose a 1,200km wide rock 39 light-years from Earth.
My theory is that W-Y were still interested in the Xenomorph, but had lost it's location. Hadley's Hope was created, officially as a terraforming colony, but with the underlying objective of re-acquiring the derelicts' location. Finding Ripley and her testimony pointed them to the location, which set off the events in Alien.