Have you done silver over white before? In my experience, silver over white tends to lack depth. Gloss as a base coat also often lacks depth. For good metallic effects I usually go for a satin or matte black or red base coat.
And yes. Always do a test piece when combining new paints - you never know what's going to react!
This was the first time I've ever used white as a base coat. My typical setup is a grey or black primer followed by black base coat. I've found that gloss gives me the best results for steel, silver, or chrome type paints. I made the mistake of not testing this time because I use the exact same brand and type of pain, only in black, and never had troubles with it. It's a good reminder to always test new paints before applying to the actual model.
I'll give the matte finish a try again to compare results. I have the head ready with a black base coat this time, so hopefully I'll be taping and painting trim in a few days.