2: A 1.5 cu. ft. upflow calcite is rated for up to 3.2 gpm, but will flow much higher. The effectiveness of the neutraliztion decreases as the water flow exceeds the systems rating. For your well, a real water test and temperature are critical to properly size a calcite system. That being said, for most regular houses up to 3 bathrooms, a 1.5 Cu. Ft. calcite system is adequate.
3. The amount of hardness that the calcite system will add is based on the pH, flow, temperature, etc. There are too many variables to make any accurate guess. The hardness only needs to be removed if you desire soft water. Hard water will have some negative affects on your appliances, water heater efficiency, etc. Submit your water test and I am sure we can get make some recommendations. If your well hardness is very low, then a couple grains of hardness that are added by the calcite may not be worth treating.
Personally, I prefer the water I use to be below 3-5 grains. Anything above that and I recommend a softener.