Well, first off, adding a second story doesnt really add space, in fact it takes away from it. Because you have to have a ramp up to it, plus support underneath so it doesn't cave in. Second, fleece is definetly the way to go. Third, I would go with a 2x4 if you wan't to have two in the future. Just make sure you read up on quarantine and introductions.
Lastly, I have three
cavies currently, two sows in a 2x4 and a boar in a 2x3. Once we get the boar fixed I want to extend them to have a 3x5, i think. perhaps maybe even a little bigger if we can manage.