I want the gloss effect but i'm afraid it will cause the decor to either "melt" or fall off the cake, or do something weird, any tips?

I steam every one of my cakes. . .it should be fine. . .don't hold the steam too close to the cake and be careful over hand painted parts as it could cause it to bleed. But over all just hold it about 6 inches away and steam evenly over the cake. It will melt all the powdered sugar and clean it up for you.

If you want it to say super glossy, or if there are pieces you actually want "shiny". . I brush them with vegetable oil, or you can brush the cake with vodka.