A. B. West
Why aren't you PLAYING a game?
1) When moving a fleet through an unoccupied planet *must* you place one of your empire tokens, or can you choose not to?
You do not have to claim a sector.
2) When exactly does the pirate steal? As soon as the merchant action is taken or as soon as that players turn during the merchant action begins?
A pirate steals any time during his merchant turn.
3) Can the pirate steal a metal if their storage is full? If so, can they sell it or must they discard it?
Technically you can. Because you can use the pirate ability any time during your turn, you could sell a metal at the start of your merchant turn then steal a metal to put in your container.