if conditions for growth were right than yes, it'll cover everything.
scrap off what ya don't want. (this does help to spread it around some though)
Fish being freaked at reflections, this would probably be on a fish by fish basis. Some species willd efinatly be more aggressive to seeing "another" male or so in the tank with em, but some may not care. And even some of the more aggressive ones may not mind after awhile, while others may ram the glass till their dead.
The only reason I would think people would stay clear of mirror on the back would be more lighting oriented, I hear alot of people mentioned adding a mirrored backgroudn to help with light reflection, mirrors are actually not a good reflector but just for viewing purposes, I don't see why not, as long as you watch the fish carefully and remove those that just won't adapt.