We quarantine new buns for two weeks, any immediate problems usually show up within this time. I would never risk letting a new bun near any established healthy rabbits, it's just not worth it. We have lots of rabbits too, but it's just something you have to do. I couldn't live with myself if any of the others became ill due to contact. Hands are washed and hygiene standards taken into account at all times. This is a must if you are going to do things properly.