Awesome! So feel free to check all the wallpapers i have here in my itch io page ^^ I also have a great amount of assets for shmup games too including backgrounds ^^
I saw them and I very much great use for them, and as for your enemies assets I gotta know if I'm allowed to turn them into allies for the player as the humans will be the main enemies in my game.
if you are asking about the enemy ships i have done in my assets, yes feel free to use them in any way when you adcquire the assets you can use them as you want ^^