The formation can be either: 1) Three players in the front row, four in the second row and one in the third row: loosehead prop, hooker, tighthead prop, flanker, lock, lock, flanker, eighth-man or 2) Three in the front row, two in the second row and three in the back row: loosehead prop, hooker, tighthead prop, lock, lock, flanker, eighth-man, flanker.
Source: wiktionary
If a player is on the ground you can try to rip the ball from him, but you can't go off your feet and dive on him. The ex-All Black eighth-man, Murray Mexted (who is a commentator today) described this rule best in one of his commentaries: "I think this rule is hard to apply because the moment you see a man on the ground the first thing you want to do is go down on him."
Source: wiktionary