Aliens shooters are extremely stale. We've already had AvP Jaguar, AvP PC, AvP2, AvP 2010, Alien Trilogy, Aliens Online and Alien Resurrection. In each one, the marine experience is pretty much the same. Colonial Marines looks no different in that regard.
If you're going to make a game using the Alien franchise, you need to do something interesting with it. The Alien RPG had potential but it was cancelled. CM doesn't look compelling at all.
The game looks great period, regardless of your opinion. There have always been hybrid aliens, and multiple kinds of Aliens, where you get this random fucking concept that there is 1 type of alien is beyond me, there are Warriors, Queens, Drones, Facehuggers, Aliens from dogs look different and move different. Prometheus introduced the fact there is even more evolution in the time period between it and the original Alien movie as well. It's been in all the movies, it's canon. The bottom line is, you don't dictate what is canon, Ridley Scott does. He had plenty of input on this game working WITH Gearbox, so if it's in there, guess what, it's now Canon.
You probably shouldn't mention Prometheus in relation to Alien canon. Prometheus had so many logic holes and inconsistencies that there's no way to tie it to Alien without breaking everything. Secondly, of the four Alien films (excluding the AVP films), Scott was only involved in the first one. There were no colonial marines in Alien. James Cameron's Aliens is clearly the inspiration for Colonial Marines and thus, his input would be the most relevant. However, he hasn't been involved with the game at all. Also, Scott didn't even provide input on the game either. Some Gearbox people were apparently invited to look at the Prometheus script but that's it. Since Prometheus was terrible and has an extremely tenuous connection to the Alien franchise, getting a sneak peek at the script doesn't mean much.
In conclusion: Prometheus sucked and Ridley Scott is not the sole authority on Aliens canon.