These sprites kinda remind me of Galaga and Galaxian. Not bad.

(Boring technical info: though a real NES only gives sprites 3 colors. Some of the critters here take more, which would require overlaying two sprites  -- something that, while used in games like Mega Man, wouldn't be used in an old shooter due to sprite limits (64 on screen, 8 per line).)

(Boring technical info, part 2: not only that, but such games (like Space Invaders or Galaxian) would, surprisingly enough, use background tiles for enemies and sprites for background stars, as fitting even four 16 pixel wide enemies on a single row would completely fill that row's limit. That still makes 3 colors the limit, though; while background palettes do have 4 colors, one of them is shared among all four as a "background color".

Still not bad. Don't worry. With some small recoloring, all of them would fit fine.