I don't think it's a bad idea. I think anything is better than shelter life especially when they will have inside time and lots of love. I do agree with Julie and think that two would be better than one so they would never be lonely. I do know a few of the shelter cats go crazy alone. They have spent so much time in the company of other cats.
It would really be great if you could eventually trust the cat and dog together but at least you would have a place to keep it safe until you were sure of how that would work out.
Do your winters get very cold?