I really wouldn't be too worried about the FIV test at this point. False positive are common. You cannot have a false negative.

Even if she would test positive, unless she or the other cats are aggressive, I really wouldn't worry about it. I'd be more worried about having a FeLV+ cat around other kitties, since that's easier to spread. Many FIV+ cats are just carriers, and are not sick. Not sure if that's the same for FeLV+.