#1 - A pedal is simply longer for more leverage, and also thinner to fit between the pedal and crank better. Also it's normally 15mm.
#2 - Mostly metric.
#3 - This isn't really a question.
#4 - Use grease to prevent the bolts from rusting in.
#5 - Use grease, almost anything works (for threads anyways).
#6 - Crank Brothers pedals have their own style of cleat, but it compatiable between all of their mountain pedals.