i'm not so sure either. i think he's a regular adult, except that you get him like you would get an old timer. i think you get an adult, and then he changes into Bill.
my dream tamagotchi shell would be neon green, and it would have a picture of a memetchi sleeping on it. but most importantly, the shell would be made of a jelly like material, that way it wouyld never break. And the background would be a rainbow swirl
you can have a baby with an elder but it needs to be with another elder, like J_tamagirl said, and sorry, but I don't know if you can debug australien tamas.