I have always heard that unless you're adopting from a trusted rescue or other private party where you know they weren't exposed to anything, quarantine for the three weeks, unless it seems like the guinea pig starts to be dangerously depressed. Anne Arundel has a small animal room, right? That's better odds, but since it's a shelter I would still play it safe. It could be three weeks for you to get to know her really well before she gets distracted by her guinea pig pals
I think people say three weeks because that's how long it takes a URI to manifest.
If she does seem really down though, you could take her to the vet for a check up and see what the vet advised, and maybe get some ivermectin to treat them all?
I hope that was helpful!