Can you play Big Two with multiple decks for more than 4 players?
Yes you can play big 2 with more than one deck.  I would recommend two decks for 6-8 players, three decks with 9+.  (5 player is tougher, two decks is too many, one is maybe not enough, but I would err on the side on playing one deck for five players, unless you want a particularly wild game).

Higher level hands such as 5 of a kind should be allowed.

Regarding tied cards, there are several different rules variants of how to play the game.  Some allow it, some disallow it, and some play that playing a duplicate card causes the next player in line to be 'skipped' and lose their opportunity to play.  Experiment and figure out which way your group prefers.