i've done a little work on this
... so i can answer some of the questions. first, there are all kinds of salt crusts. the one i prefer is a simple one -- kosher salt moistened with just enough water to make it like slushy snow. other people, such as my honorable friend mr. colicchio, prefer egg whites. (FU Edina)
i don't think it matters to the fish. the extra fish will have a minimal effect on baking time. i'd plan on a little longer, maybe 3-5 minutes, but maybe not even that much. there are two ways to tell when done -- use an instant-read thermometer (the fish will be done at about 125 degrees). or you can brush away the salt near the fin and the fish will be done when you can pull a fin bone out. i might prepare the salt in advance, but i wouldn't prepare the dish until just before. i don't KNOW that it would salt it, but it might. and since it takes about 45 seconds to put the thing together, why risk it?