I've heard of people keeping breeder colonies in the same room with their snakes without having any problems at all...so I'm sure having rats in another room of the house won't be an issue.

As for switching, I can't speak from experience yet, but can share what I've learned vicariously through everyone else: Eventually, your ball will be too big to make an effecient meal out of a mouse. At that time, a rat would be a more appropriate sized meal. That being said, it is sometimes difficult to switch a finicky eater over from mice to rats...so people will often do it while the snake is still young and not quite so set on a certain prey item...which is why they may go from a single mouse to a baby rat.

You don't HAVE to switch at all. Some people successfully feed adult balls multiple mice. There is an ongoing debate on which method is more beneficial and I have yet to see any compelling evidence for either side. It seems to me more an issue of personal preference and convenience.

Some with more experience than me may have more specific things to say about it, though.