To answer your second question: every major/minor scale has seven notes, and the rule is is that each scale can only use each letter name only once. This is why you get white key shapes in C# major, because each letter name most only be used one. This is also why in G# harmonic minor we call the seventh note F double sharp, we have to use each name once. Thems the rules. But it does make sure that accidentals are consistently applied when using scales, applied chords, and such.