another possible option is to stick the decals on and let them dry
then, after they dry, but before they start to peel off, cover them with the 2 inch wide clear tape intended for closing up shipping boxes. They are in stores near the padded envelopes, if they have any.
I use that tape as a wrap around every bottle of paint I have,,,,,to keep any paint spills from wiping out the name and number on the label.
None of them have peeled off, and I tested one with some blue paint held up against the tape edge, and it won't let paint wick under it
just an idea,,,,,,,,it might work,,,,,,and at the very least, you would be smoothing down tape instead of worrying about tearing decals when you smooth those down
Rex