Lise-
Our house is pretty securely made... the only hole we've had problems with seems to be one made for wiring under the kitchen stove, that just empties into a closet.

DGF says that rodents left to themselves will wander around the house, but generally come back. I don't trust it, but have no real reason to disbelieve her, either... she lets gerbils run loose, and they're maybe half Gebo and Shyguy's size!

G and SG live in a ferret cage, separated (they fought too much when we had them together, and it would've been quite expensive to get them fixed...) and come out every day for an hour or so to play. Gebo helps with bike maintenance (sleeping in my hood or running up and down my arms while I'm cleaning the chain, etc) SG is too neurotic to allow me to focus on anything else, but loves his play time, anyway.
When they're a little older (George and Steve look like just lil' bebes!!!) they get really cuddly. Gebo will just sit there sometimes like a furry potato and let me rub his neck, cheeks and ears, bruxing his head off.