So basically, given that all Genasi are on their planes from the moment they are conceived in the womb, and are born on their plane, all fire Genasi instantly die from fire damage because none of you put immunity to non-magical fire.
The fire Genasi NEEDS immunity to non magical fire or it doesn't live as a race.
It's not as if it's going to be attacked in game by non magical fire.
As for all being human, just fluff it so that they look like whatever race gave birth to them.
