The only thing I'd offer about decals is: paint as it dries/cures it sends off gases/solvents. These sometime can show up under the decals as tiny pinpoint bubbles
or they can attack the solvent that is used in the glue of the adhesive.
Now - basically, all you have to do is wait for the paint to thoroughly cure. I've always painted over the top of decals with great results.
Normally when I paint a tank, I'll prime it, wait a week or so, scuff it and shoot a primer/sealer and proceed with the color. Then I'll apply a coat of clear and let that
cure/dry a week or so. Then I'll sand it flat with 600 and apply the decals and shoot the final coats of clear. Sand everything flat again with 800 - 1000 to get rid of
any ridge the decal might cause and then I'll polish it.
Now, I realize that it's probably a bit of overkill to wait so long between coats of primer, clear etc but I've always been warned of paint shrinkage after sanding etc.
After all that - YES you can paint over the decals.