STO apparently have a problem drinking from water bottles as youngsters and often get a mash of fruits to help them get their water intake. There is food specific to opossums online which would make up most of its diet, but I don't see it being sold in stores anytime in the near future.

You'd really enjoy rats. They are very smart and fun to interact with. They are also much more easier to care for than exotics. I think you'll find that once you are out on your own, you'll have less time and energy for all the pets that you'd love to have right now. Life changes, keep this in mind and wait a little while after you move to decide if the time is right for more pets.