Re: How to make gloss.....matt ?
Is the color you are using ONLY available in gloss as opposed to flat? Most flat paints are easier to paint then gloss ones. If it is acrylic (such as Tamiya brand) then a flat base can be added to the paint to make it more matte. Otherwise, a clearcoat over the top is about the only way to go to my knowledge as I am not sure what can be added to enamel paint to flatten it.
As for durability of a clearcoat, that will typically depend on how fast it is shot on. If somebody tries to soup down the surface in one pass, it may have some cracking issues. If it is built up slowly, it works better. Beauty of flat clear coats though is that typically even a light first pass will do the job of flattening the surface. It is a lot easier to flatten a glossy surface then it is to gloss a flat surface. Try using gloss on flat paints and it usually absorbs the clearcoat like a sponge, meaning more coats have to be used.
As for bonding issues, a hobby strength lacquer clearcoat such as Testors Dullcote will bond just fine with enamels and not be too hot to damage the paint. In fact, many of the clearcoats I've seen sold with enamel paint lines seem to be lacquer based as opposed to enamel based. I have seen enamel based gloss clear coats, but not flat enamel ones.
Of course, gloss has its advantages. For instance, if you are putting decals on, they will stick and look better over a gloss surface then over a flat one. Then they look even better when clearcoated over the top.