Nikon(G)-EOS fit for Nikon G Lens and Canon EOS Mount Camera ,your cannon 80D is E mount camera, so the adapter ring is fit for your camera 

Yes, pretty much any camera with a standard mount 

I would say yes with confidence. I've had 10+ tripods, 6-7 cameras, 6-7 bino's and 3 spotting scopes over the last 30 years. Everyone one of them use the same screw size. I don't know the size and I've never asked. They have always fit for me. Seems pretty standard to me 

They fit all cameras l think. 

FD-EOS fit for Canon FD mount lens and canon EF mount camera body. canon 60d is EF mount, so the adapter ring can work with your camera, but you need to check your lens mount. could you pls let us know your lens specific model? service(AT)kentfaith(DO NOT)com, Please note: It is not same between Canon FD mount and EF mount, this adapter ring is only for Canon FD mount lens, please contact us if you want to purchase for Canon EF mount lens.