If you get the UNIBIOS and put it (neogeo.zip) into MAME and FBA_LIBRETRO you can play whatever version of Metal Slug you prefer. No matter if it's AES, or MVS. You can even switch it. So...

Without UNIBIOS

Place the normal NeoGeo Bios into the neogeo, the mame, or the fba_libretrofolder Put the Metal Slug Games (zip) into the preferred system (I recommend fba_libretro) Have fun

WIth UNIBIOS

READ THIS Do steps 1 to 3 from above 🙂