It is standard practice for the feet to leave the ground ever so slightly, we're talkin' a hair's width.
Depends on your goals - both are very powerful movements though.
Power versions will often work better in a well-designed athletic training program.
That's a big animal. You won't want a strictly Oly program for athletics though, but rather mixed in with a myriad of other things. The other suggestions are good, as is Artie Dreschler's Weightlifting Encyclopedia.
Hey, you learn by asking questions
What sport do you play?