I'm sorry, I thought it would be easy to find out about mixels but I've been looking for like 20 minutes and I didn't even find a website or video that explains it! Basically the idea is not to mix resolutions. I read this in Michael Azzi's "Pixel Logic" book (page 21), you can find it on gumroad if you're interested. It says one should never mix resolutions. Basically all the pixel art should fit on the same "grid", there shouldn't be a sprite that looks like it was resized 2x, if that makes sense. Another way to say it is all the outlines of the pixel art should have the exact same thickness.