I am so glad to hear you chinchilla is recovering. As far as the baby I remember that when I got my chinchilla, Kulu, at three months old, he was still pretty small (300g) and able to fit into places that an adult could not although he did well in my big cage although it took a couple of weeks for him to learn how to use the upper shelves. So I know 300g is safe. That's about all I know there.