Ken, thanks for the lead to printable water-slide decal sheets! Back when I did some, Krylon Gloss Clear sealed the inked surface - couple of misted coats then a minimally wet gloss coat. When dry, and starting, again, with a mist coat or two with your finish material to prove compatibility and prevent dissolving parts of the image
When doing mine, a few years back, I noticed that "white" sheets were not very opaque - strong colors underneath showed through as clear 'shadows' in those colors. Also, it could simplify cutting to the "content" if you could print a close match of the color under the decal around the actual image(s). That's less problem with mostly a white base color. Haven't looked at your suggested source yet; do you know if their "white" material is more opaque?
Best remedy then seemed to be to mask for and paint white for under the decal colors. I did use the translucency once, on a yellow silk-covered, clear dope finished wing. Decal surface color included a medium blue. Sunlight shining through changed that to a glowing green when the model was high, or turning - changing as the light hit it differently.