1. You'll need a cassette lockring tool and a chain whip to remove your old cassette. When you install the new one, don't be afraid to clamp down on the torque. A tight lockring squeezes the cogs together and minimizes damage to the freehub body.
2. While it is possible to source some individual cassette cogs, it's generally more cost efficient to replace the entire cassette.
3. Campy and Shimano cassettes use different splines so they can't be interchanged. SRAM cassettes are splined the same as Shimano. I'm sure some folks will argue the merits of SRAM vs. Shimano cassettes but I won't.
4. I seldom worry about the intermediate cogs sizes. Most cassettes are designed by the manufacturer to keep the steps between gears as equal as possible. Shimano mega range cassettes are an interesting exception to this rule.
5. New chainrings have teeth that are rounded on top. Worn chainrings are noticeably pointy.