That's right--you had one of my really early sets before I figured out what protective coating works best. My decals don't suffer from that nasty cracking and peeling anymore.
You can get away with applying the decals to your valks without laying down a glossy clearcoat if you use Micro Set and Micro Sol in tandem. Heck, you should use them anyway but they really help the decals adhere to the bare plastic. As for a top coat, if you are going to handle your valks on a regular basis, I would suggest coating them because no decal (even professional ones) are designed to stand up to repeated handling. If you plan on leaving them displayed a shelf, then you won't necessarily need to coat them.
If you have access to an airbrush, I'd recommend Future Floor Polish for the glossy base coat and Micro Flat for the top coat. If you need to go the spray can route, Krylon Crystal Clear is a good base coat and Krylon Satin is a good top coat.