Hello, i have a quick question. I play on an emulator and i'm getting an xbox one controller very soon. That controller has instead of just an a & b button like on the gba, 2 extra buttons above it. Is it okay if i use one of those two buttons as select so i can toggle between missiles and supers easier?

No rule against that.

Hell I use a keyboard which is completely different. The keys that correspond to the buttons are nowhere near what the layout would be for a normal controller.

I personally use R1 for missile toggle and R2 is R, while L2 is L.... but pretty much everything else is the same.

I am a little weary of mapping an input to multiple buttons since it's an emulator only function, but I can't really think of many ways to abuse it, so it would likely not get banned. The best I can think of is being able to map two buttons to down so that we can morph faster. It is probably easier to do a heal glitch setup with this.

Pressing down with 2 different buttons is possible with the Game Boy Player too, if you feel like dealing with multiple controllers.

(But yes, please don't do anything stupid with button mapping)

I think that there's a bit of a delay when doing that with the GBP. This is something I'd have to look into though.