I think I know where Luigi's side-B green missile originated from...

[mistersl56]

My best guess for this attack's origin is Super Mario 64. There were cannons everywhere which Mario could hop into and become a human missle. Luigi wasn't in the game and so to make up for that, they gave him this move.

Or it's probably a coincidence.

 

[Luigi#1]

It sorta makes sense, but considering Luigi wasn't in the game (until the DS remake which came after Melee) I really am unsure.

 

[FangedSniper]

I always liked to think that it was him trying to imitate a bullet bill...