21 grains per gallon is fairly hard, 42 of course is even more so. I don't understand the comment about bacteria, because no softener, RO system, or filter of any kind is rated to handle water which is not bacteriologically safe. With the chlorine content, it is probable that the water IS safe.
Many softeners are rated to handle a certain iron content. Check the specs on that one.
As for the softener: Let's say family of 4, uses 300 gallons per day. Lets take a softener rated at 39,000 grains. 300 X 21 is 6300 grains per day. 39,000 divided by 630 means about every 6 days it will regenerate. This is a reasonable number. If they use more or less than the 300 gallon per day in my example, the regeneration would change. If the softener uses 4 pounds of salt per regeneration, that gives you an idea of your salt cost.
Make sure that the softener is installed so it does not soften water to any irrigation system or the back yard hose bibb.