Yes, you can intermix the two modes happily without a problem. In fact, this is something that came to a surprise to me after i've added the first bits of the new stuff...i tried to mix both and it worked right from the beginning...

About the textures...it depends. You can always do a setTexture(...) on a compiled object without a problem. You may as well modify the u/v-coordinates if you've compiled it as "dynamic", which is slower but still faster than cpu processing, at least on my machine. What you can't do is to modify the texture of a single polygon. The compiled object simply won't reflect this change. For a decal, i would go for extra decal polygons on top of the scene's geometry anyway (Robombs does this for example).
And finally about shaders: I guess so. I've no real experience with shaders, but as long as you stay in the pipeline for compiled objects, it should be possible. Maybe i'll add some hooks later that allow people to hook right into the pipeline and do custom stuff...