There's a lot of theories floating around out there of why the aliens look different in ALIENS. Some of them I can recall off the top of my head are that they were produced from the Queens eggs so they're actually different from the derelicts cargo, or possibly they 'molted' their dome as they matured since the alien in Alien is only about a day old max whereas the ALIENS are there for weeks.
With that being said though, Newts parents and those that went into the derelict and got infected as well were exposed to the eggs from ALIEN. So, does that mean there were some chaps running around? Did they start building the hive? Is that where the 'drones' lore comes from versus the warrior aliens?
I think it would be pretty interesting if the chaps were the ones that started taking over Hadleys hope, maybe one evolved to the queen, maybe the chaps were still in the hive but doing their drone work, or maybe the did evolve into the 'warrior alien'