Clearcoat will not harm the decals nor hurt the silver paint (or any paint for that matter) In fact, they will help, seal the decals in. Most WW2 thru Cold War eras, silver were never shiny. Flat clear will make it more realistic.

I always use Micro Set and Sol and follow up with a final coat of clear. Just don't spray clear on unmasked canopies of aircrafts.