Cycle Stop Valves are either Stainless Steel or Plastic, and acidic water is fine. Most of the first CSV's installed 28 years ago are still working, so really can't tell you how long they will last. Have a picture from a customer who had an old Hydroservant type valve, which is similar to the CSV, and his pump system finally gave up the ghost after 52 years. That is exactly why Red Jacket stopped making the Hydroservant, and why Franklin stopped making the Aquagenie they bought from Jacuzzi. It is also why there is so much false propaganda against the CSV. Pump companies want you to have an expensive and short lived Variable Speed pump system, and will say anything to keep people from using a CSV and having their pump systems last 50+ years. Written warranty on a CSV is one year, but we would never hold to that for anyone who had a problem many years down the road. Most failures are from the CSV being packed with sand or let freeze and burst, yet many times we still issue a curtesy replacement, as we have so few warranties we can do that.
Your 68 gallon pressure tank holds about 15 gallons of water. You will have to use this 15 gallons before the pump will start, so no the pump will not start when flushing a single toilet. However, most CSV's are used with a 4.5 gallon size tank that only holds 1 gallon of water. With the small tank the pump will start when flushing a single toilet. However, the CSV will keep the pump running continuously until you are finished using water. That means you could flush a thousand times in a row and the pump only cycles once. Your water doesn't come from the tank, it comes from the well and pump. The tanks only purpose is to limit the number of times the pump cycles on and off. When you have a Cycle Stop Valve to do that for you, you don't need much of a tank.
The small tank used with a CSV is just a mechanical timer. The CSV will fill the tank at 1 GPM, the tank holds 1 gallon, so you get 1 minute of pump run time after flushing or shutting off any faucet. In this way the CSV is making sure you are finished using water before the pump is shut off. The small tank will cause the pump to cycle for a toilet flush, but the CSV eliminates all cycles for long term uses like showers and sprinklers. Even if the pump cycled for every toilet flush, which is doesn't, it will still cycle much less overall using a CSV than not.