if the camera comes with a tripod ring, just use it. Although the camera body should be able to withstand the weight of the 70-200 2.8, not using the tripod ring will mean you have a very unbalanced set up on the tripod which will mean your lens will keep drooping down towards the front.
Easily. There is a higher chance of your lens flopping downwards when you lower the friction on your ballhead though. I am sure even the 5D Mk2 can take a 600L although stability is another issue.
If your lens comes with a tripod ring, just use it. If the tripod ring is optional, I would only recommend buying it if you shoot long exposures <30s