Shotgun:
Press Triangle, Square, Triangle, Left, R1
at the pause screen to start with a shotgun.
Level skip:
Select "Arcade" at the main menu, then aim at the black border of
the boxes with "Beginning" or "Warp Ahead" to begin farther
into the game than normal.
Rebel Kronn warrior:
Shoot the first three S.T.A.A.R. team members and nothing else at the
beginning of the game. After the first human is shot, you will not be harmed by
any gun fire. After the third human is shot, you will transform into an alien in
the main hallway of the first level. You will have infravision when playing as
an alien, which allows targets to be more visible.