Some questions about pet rats.

[Dracen]

I've been considering getting some pet rats. Before I do, I've got a couple of questions. I've done some research and read a really good article on them, but it raised a few questions, so I'm gonna ask some rat owners.

I've read that rats need to be out of their cage for an hour per day. Well, before I did that, I'd have to rat-proof my room, but I'm worried they'd chew on the books that are currently being used to ferret-proof the room (don't worry - the ferrets and rats would never be out at the same time!). I was wondering - would letting them out in a plastic ball be fine? If so, what size ball?

The page also said that a clean rock or brick is good for rats to wear their claws down on. Something like this would be ideal for me - my mom doesn't like rats and would not hold one to help me clip its nails like she does with my ferrets. Would a rock on its own be enough for rats to clean their nails on?

Would a rat bite, especially a young one? My ferrets are still biting, but they're young as well. I imagine there would be some biting in the first couple of days as they adjust, but how long can I expect this to last? Can rats be litter-trained, or no? Thanks! =)