I'm honestly not sure if you can get away with one tank size for life with Ball Pythons.
A 30/40 Gallon is too large for a neonate, and a 20 Gallon is too small for an adult. Especially for an adult female.
Personally, I'd get the 20 Gallon for the neonate/yearling. You can wait until it gets a little size on it, and then replace it with a 40 Breeder.
The 20 Gallon doesn't have to go to waste in this hobby! You can get a Corn Snake, Crested Geckos, Leopard Geckos, etc.
