1. You can probably go pretty small, as clowns will absolutely not stray very far from the anemone. The bigger question is how big will the anemone grow. And if you want to keep any other fish or corals in the tank.
2. If you're talking something simple like a RBTA, I would say it's about as simple as a soft coral tank. Now if it's an anemone like a carpet, haddoni, ritteri, magnifica (etc) then I've heard those are more difficult to keep, I really don't have any experience with them though.
3. Zero dosing necessary, they don't make skeletons, hence don't use it up. Coraline algae is about the only thing that I can think off that would consume calcium and alkalinity.
4. Not very often, as I said, soft coral tank. It really depends on what you feed the tank, if you overfeed the clowns you may want to do more water changes, but if it's just a couple clowns water changes should be extremely minimal
5. I'd say yeah, water changes are to maintain the tank.