I'm sorry I dont remember what size the bolts were. The double sided tape can be bought at most general hardware stores (home depot to wal mart) and probably your autostores would sell it too (probably cheaper elsewhere.) The tape is needed to help firmly attach the spoiler and keep it from rattling. It's sort of a thin foam that will help hold the spoiler tightly in place once you bolt down the spoiler. Otherwise, its just going to be plastic (whatever the spoiler is made out of) directly to metal of the body, that will rub and rattle.
Ford used the same type of stuff when they installed them on SVT's. You shouldnt have any issues with rust, they wont be easily exposed to the elements.