You can not. I don't know why. It wouldn't have been that much trouble to make them playable in the player one slot. That is one of my gripes about this game. You can switch to Luigi in NSMB and SMBDX but not this? Your best bet is to stick Mario in a bubble and pretend like he doesn't exist.
I will play as Mario but I like to use Yellow Toad on Fortresses and Luigi on castles. Thats what I do.
Aha! I was looking for that code...now the question is, is there a modifier for red toad, since the ai kinda acts like a PC? Then again, if he got a powerup the game would probably crash or something, huh?
In single player, if you die 8 times you can activate the Super Guide. And then you press + to exit the super guide, you will continue to play as Luigi for the rest of the level.That's the only way to play as another character in single player.