This is mostly just a lit of stuff I forget how to do and look up constantly. If you find use of it, great.
Moving Symbols between pages
Symbols don't have to stay on the "Symbols" page. You can move them to any page you want, but you have to drag them from the Layer's list and onto the Page you want to move the original Symbol to. Dragging the Artboard from the Canvas doesn't work.
Overrides are size-dependent
If you want to create icons in two sizes, 24px and 48px, and you want to be able to override the colors, you can create four Symbols: 24px red and 24px blue, plus 48px green and 48px yellow. When you want to change the 24px from red to blue, you'll only see those options, not green and yellow.