Unlimited credits:
Press C-Left(2), C-Right, A, B, C-Up, C-Right at the title screen. Enter the options menu and highlight the "Continues" selection. Press Left until the word "Freeplay" appears.

Fight as Grox:
Press Down, Right, Left(2), Up, Down, Right, Up, Left(2) on the character selection screen, then choose any fighter.

Fight as Exor:
Press Left, Down(2), Right, Left, Up, Left, Up, Right, Down on the character selection screen, then choose any fighter.

Random character select:
Press Up + Start at the character selection screen.