Beware, mice rarely come alone. I got overrun in my shed because I thought 'awww' at the first one and I ended up with tons of them as they bred under the shed and then migrated into the house - I got tough and started catching and/or killing them, in one day I killed 40 mice in one room. And in the end I rehomed my guineapigs as it got too much for me. Now I have a phobia of mice, I hate hate hate them!!! Mice can carry disease I believe that can affect rabbits, but I've not heard of anything they can pass to guineapigs. But my advice to you would be to put a trap down, humane if you prefer (I found adult mice didn't go in the humane traps, but babies did). But you need to take action now before they take up residence for the winter with their families in tow.