I don't know about putting the girls upstairs and the foster boys downstairs. It might work, but I'd feel incredibly guilty! Although, that would limit the peeing to the 3rd floor, and I could hold out hope that the boys would be well behaved down here.
The more I thought about it the more I would try this. The 3 girls are used to each other, they did not pee before George, and no one else seems to have a problem. I'd forget the guilt and give it a try. If you have less clean up, guilt will DISSAPPEAR!!!