SM already said Isolation (I can only really speak to the game, haven't read the book, but the game's influence is all over Romulus) and Blue already said The Cold Forge (and, by extension, I'd add Into Charybdis to that recommendation just 'cause you can't really have one without the other). The two Alex White books are easily the best of the Alien expanded universe novels.
But with those already said, I'll throw in a bit of a wild card one: Aliens: Phalanx. It only really meets one of your criteria (a mostly younger cast of characters without a clue what they're dealing with who grew up/live together on a world that's really all they've known up to this point in their lives) and... to get into more than that would be to spoil the fun of it. It does have a neat little tie in to The Cold Forge, though, so I'd recommend reading it after TCF if you do end up going with this one.
