Silvering is caused by tiny air bubbles under the transfer. Because the matt surface is rough on a microscopic level it is more common on matt surfaces. Gloss surfaces are smoother so the chance of it is greatly reduced, but not totally eliminated. Using a setting and a softening solution (Micro-Sol and Micro-Set or equivalent) can also help reduce this.
There is nothing that stops you applying it over matt surfaces, bit I always apply gloss first. As I come from a war gaming background I tend to gloss the whole model as this is more hard wearing for handling, then transfers, then matt or satin for desired final finish. If I weather I often seal just the transfers under a gloss, weather and then do the final matt/sating coat.
Leigh