I'm playing on my evil acount right now on HC mode and I was with Boone going to get the Guass rifle from the aliens when he was gooitized by a fiend with a plasma rife. now being evil this doesn't effect me in anyway (besides one less companion) but i want to know, is it just plasma that can kill? or is it an update or HC?

With hardcoe mode turned on your companions are not essential, they can die. With hardcoe off they're essential and only go unconscious when they're health reaches zero.

You won't get the unique Gauss Rifle if the aliens are there, only the Alien Blaster. If you want the Gauss Rifle don't take the Wild Wasteland trait.