I would go ahead and get the 20L right away, you will save some money since you will need it in a few months anyway. If you think about it, when the babies are born they have a whole world to travel, so a 20 gallon tank is definitely NOT too big! Just provide lots of hides, smooshed toilet paper rolls half buried in the substrate are favorites, as are bandaid and toothpaste boxes. Not terribly pretty, but they are perfect for the wee ones that will outgrow them in a few months anyway, save your money for the bigger hides if you want a clean look, they will be in them before you know it.