On clear coating: I've shot clear over white decal paper before (usually with Krylon Crystal Clear, flat) without any issues. I like to apply my decals to the base coat. Some people advocate for brushing on some clear where you wish to place the decal, but then you risk trapped solvents creating bubbles. If using water-slide decals, let it dry twice as long as recommended, just to be safe. Once your decals are in place, apply multiple coats of clear, following the instructions on the can. I have trouble getting some decals, especially water slide decals, to stay adhered to matte finishes.
One problem with decal paper is that you can't really use the transparent paper on black, or other dark colors. The decal either won't show up at all, or the colors will be muted. This sucks for when you want to do text or something else that's too finely detailed to cut around. I'm starting to think that laser cut stencils are the way to go....