Not sure if you do Miltary or Aviation models but applying decals without a gloss coat first you will have major silvering, not sure how you can have good result applying decals on a matt finish base, would love to see some of your work.
This snowspeeder is covered in quite a few decals and no gloss coat. The paint is Tamiya flat Sky Grey mixed with Tamiya Flat White. I did brush paint matte varnish over the decal itself to seal it, but I didn't cover the entire model in it. There is nothing between the decals and the flat paint, however.