Glossy decals!

A flat coat will really help to tone them down and bring them to the same sheen as the rest of the model. So no, you are not out of luck. Keep in mind you should apply another glosscoat after the decals are on to help blend them in further and to seal them. Then do your weathering and then comes the flat coat.

I usually get by okay just with the flat coat.  If it is a flat finish subject, I always do an overall  flat overcoat after decaling.  I may do that coat before weathering, sometimes after weathering, depending on exactly what types of weathering I will be doing (oil and fluid leaks, for instance, should be somewhat glossy).

Good tip, Don.

This is my plan:

Gloss clearcoat, do decals, flat clearcoat, weather my kit, and finally, a final coat of flat clearcoat. Done.