For games, informal might be more appropriate. But it's a question of style and target audience.
Yes, you can replace "du kannst" with "sie können". Don't capitalize "sie" if not 1st word of sentence. That's old fashioned for letters.
Okay I left the tutorial texts unchanged.. but 2 phrases remained too long, so I made minor changes:
1) "Sammle mehr Münzen um die
nächsten Puzzles freizuschalten." changed to "
nächsten Set freizuschalten.", not sure if people will understand Set as a valid German word though..
2) "
probier es einfach mal." changed to "
probier's einfach mal." so it
just fits in the balloon, or is "probier es mal" better?