As another piggy and rabbit owner I would agree with everything said above.
I would never risk having piggies and rabbits in the same air space.
I also have separate cupboards for their various supplies, and they never share anything.
As far as rabbits go I would say companionship of their own species is even more critical than for guinea pigs.
Based on 20+ years of rabbit ownership they rely heavily on physical contact, mutual grooming sessions and close physical interaction, none of which can be provided by human companionship.
Unlike guinea pigs my rabbits will always seek each other out, lay together and groom each other. OK our male does sometimes try to sit on his own, but the female will find him and sit with him anyway
If you can separate your rabbits and guinea pigs into 2 rooms then this will make it easier to set things up so you current bun can have free roam, and remove the stress from you having to worry about cross contamination between the 2 groups. As a single rabbit she will need a lot of space and a lot of stimulation to prevent her become bored, destructive and aggressive.
Please consider contacting a local rescue with a view to finding an older, neutered male as a companion which will reduce the time she has to live alone.
Do also be prepared for a bonding to be a lengthy and time consuming project.
I have had rabbits who have been in love at first sight and i have had bondings that have taken almost a month!
They really are fantastic pets if you have the time and space to house and care for them properly, so hopefully you can come up with a solution that meets everybody's needs.