Clear coat carbon fiber?

That is a new one to me glheald, but I would think Future would work. It may be best to try on some scrap with some spare decal first, just to make sure it is compatable.

Yes you should overspray these decals whith a clearcoat.
I did it with Tamiya Acrylics, there where no problems.

Spraying a coat of future or something like it always improves the appearance of decals, especially stuff like carbon fibre decals.
On the real thing, the carbon fabric is actually embedded in an epoxy resin, so the surface you touch is actually the resin, so adding a minute thickness of clearcoat over the decal will improve realism.
Cheers,
Tree