Grr, excuse my post, it wasn't meant to be rude, and I know it was.
Mixing different sized animals in most cases is just a bad idea. I did it with corn snakes when I was 13, and lost both of them, because one tried to eat the other and suffocated because the other snake was too long for it to swallow.
When you go to the pet store (Petco usually) and you look in their leopard cages, notice which ones have missing tails. It's always the smaller ones, because the larger, more aggresive geckos have taken them. There was a guy on another board I visit who mixed an adult female with a 2 month old male, after multiple people cautioned against it, and he ended up with a dead baby, and his female had bite marks that becamse infected, which cost him 250$ in vet bills.
I don't think it's a good idea. Others on here will probably not think it's a good idea. Buy a 10g cage until he's bigger for 12$. Or, if more than one cage is really a problem, just don't get the baby. It's probably better that way anyway.