Yeah, it might be a bit too much especially if we go with the "fiery galaxies" background (the other possible background is purple galaxies so it's a similar issue).
We'll probably make the shell part of the background map, with its custom palette so it doesn't affect other maps.
Darkken, the frame with the Lavos eye open uses more colors than a sprite can have (13 colors I think?), but don't worry about it. It's perfectly fine: when we'll insert these graphics, we can just have the shell and the Lavos eye be part of the background, and King Zeal's body be a sprite that covers the eye in the idle animation but reveals it in the spell casting animation. There would essentially be a transparent hole in the body in one frame, but the eye under it will fill that hole and players won't notice the trick.
Concerning further animations, I don't have any idea... I think this wraps it up nicely. Lavos in CT didn't have a special "hurt" animation.