okay Luca, I was gonna suggest Bluetooth, but I don't know how it'd do for interference
so, here's a solution...don't know if it's the best,though. Apparently the max length for usb cables is around 15 feet, so you'd need an active extension usb cable (about $20 each) or a couple of powered usb hubs
wow-that's a lot of mac pro's.... compute much?
this depends on where you live in the world, but maybe USB over powerline....
dLan or smartsolutionskvm? I don't know if you have any other options right now
good luck to you