Technically a 20th level does not gain the Native subtype (which is a wierd thing to not change when we got 3.5, but them's the breaks), they just have a special condition that allows them to be raised and resurrected normally. The only difference this makes is that a 20th level monk doesn't need to eat or sleep.
Looking at the template examples in the Monster Manual, it seems that changing away from Humanoid removes the subtypes (Half-[creature], Vampire, etc.). I guess you could see the standard Humanoid subtypes as the same as Reptilian, i.e. they can only be possessed by Humanoids.
Trivia of the day: 20th level monks are immune to Bane weapons, as they are Outsiders that don't have alignment subtypes.