I don't believe its random. The way I think it works is this: one of the four player is chosen by wi-fi connect. When that player is chosen the wi-fi connect applies the players setting to the next match. Which means it takes the stage and IF THE PLAYER HAS ITEMS ON OR OFF! ( so set all items to off please
).
To get no cpu: When your in the waiting room, beat the stuffing out of the sandbag
until you get one brawler to pop up. When that happens don't touch the bag at all. If the time runs out and you have some spaces open and wifi connect picks you it will not put in computers (guaranteed). If you keep on hitting the bag and there is not enough players wifi connect will assume that you really want to play, and then fill up the empty spots.
Trust me, this works. Try it out, but don't pick final destination because EVERYONE picks that stage use a different stage and turn your items off.