This is a tough thing because the guinea pigs can easily be carriers of things and no amount of quarantine will catch it.  I had problems when I rescued degus and brought them into my home.  No problems with them at all but within a week my girls had upper-respiratory issues. Quarantine would have done nothing. 

Problem is you run risks every time you intro a rodent into the home.  Doing a quarantine is a very good idea, because some things they can bring in that you can see, but quarantine is no guarantee.  That doesn't mean I think you shouldn't get the cavies, just take precautions and watch your rats after bringing them in even if there was a quarantine.