That would have to be a robust desktop if I understand your intended setup.
Concerto is just a web browswer. A feed is connected to a specific URL. If you created different "devices" in the Concerto settings, you could have 8 tabs open and use 8 different URLs for each feed.
I'm not sure how well a tablet would work in adding content.
I would definitely test the concept before building this out. With the amount of hardware you would need to drop into a server to produce out video outputs, I would invest that money in some Raspberry Pis and just connect those to the tvs/monitors. You can set them up in kiosk mode if you ever need to reboot them.