I have a wacom, but I rarely use it for my pixel stuff. I do most stuff entirely by mouse, some more difficult things I draw on paper first. My first (and probably most important) thing to tell you is, don't focus on the lines. Focus on the volume. That's probably the reason your works look symmetrical, since you focus on the lines. I saw the same thing in your Frankenstein too. No human being/creature is perfectly symmetrical. I'd say work on sketching on paper with a pen first. That's where the basics of being a good artist comes from.
You do seem to have a good sense of color.