I also had, and still do have the technical manual from the movie, it is a awesome read, very well written and informative.
In it there are paragraphs of “scuttlebutt” traded among various soldiers some in relation directly to Aliens or slightly skewed versions of the same but with details confused or entirely false, a very nice touch adding depth to the book. Some of the other stories in it talk of fighting against human enemies and the tactics used against them, so the colonial marines were fully capable and tactically able to take on humans!, not so good at killing machines.
TL;DR i love Aliens! its my fav Vietnam movie!
Regarding the colonial marines being sent, It would make sense that the government would have a substantial presence in patrolling any trouble spots and a company as large as W/Y would likely have military contracts etc that could help get something prioritized if they so wanted. However from the film it appears that quite some time has passed between Ripley losing her license and Burke resuming contact with her. So it is safe to assume that W/Y did not pull strings and Burke is either acting alone when he decided to follow up and get the derelict checked out.
There were 158 colonists at Hadleys Hope and a unknown number of cattle, in which to breed xenos.
The marines that were sent to LV426 was only half the number of a standard compliment, hence two dropships and APC’s.
It is fair to assume that Burke was not in charge of 426 as the base commander would not have referred to him as some suit in a office back home, but by name. Also if Burke had contacted the military he would have likely downplayed any risk and Ripley and himself coming along was just in case as voiced by Gorman on there initial meeting. Gorman looks like he just came out of the academy during this meeting and is obviously not in command of the marines he is later. Hicks commenting on Gorman not eating with them indicates they do not know each other, additionally he does not know them by name. Compered to Apone who clearly has spent considerable time building relationships with the team. As none of the marines act surprised someone has been replaced it is fair to assume commanders are often switched around and the fact there are only half a section would cause them to suspect its a easy mission and to not expect any major trouble.
It is clear from the mission briefing with Ripley that only Hicks and Apone are focused and treating the mission with respect. It is also clear from the breakfast discussion they have no idea why they are there but believe they can take on anything. Hudson’s cockiness is surely down to how capable they are and what they have dealt with previously.
When they locate all the transmitters together they have no reason to suspect anything other than they have holed up there. Also i do not think there is any mention or reference to the aliens nesting in Aliens or the initial release of Alien, So the marines As they logically suspect they will be escorting large numbers of civvies out of the danger zone it is bad tactically but logical to send in the entire team.
No excuse for dumping ammo whatsoever!
Almost all of the marines extra ammo and weapons would have be taken out in the dropship crash, and the following APC explosion. So all of the fancy toys Hudson mentions went up in smoke. If the APC had survived it was still capable of movement despite breaking the transaxle so i imagine they would have driven it as far as they could from the facility and used it to transmit a distress call to the Sulaco and awaited the second dropship.