Solvaset is a widely used decal softener, BUT you need to test it on a scrap decal first. Some brands of decals don't take kindly to the solvent in it.
Lots of good flat, satin, or gloss coats for over coating. Vallejo works well, so does Tamiya, and both are acrylics so they clean up with water.
You just need to try them and decide which you prefer.
As with almost any products, you should try them out on scrap decals to see how they react.