In order to have all characters available for NG runs you'd have to manage to complete Mission 7 (or 8?) without ever grabbing a weapon pickup then make sure to end a mission with exactly 10 grenades. This way no one's starting loadout includes anything extra. This is a complete pain in the ass to do so I figured I'd upload this in case anyone got interested in the game and happens to have uLaunchELF or some similar method to put these files onto the PS2MC.

All characters available for NG runs.

Metal Slug (2006)

PlayStation 2