You don't size a softener based on the pressure or gpm output of a pump. You size based on the maximum gallon per minute peak demand flow rate of the house/family based on what type and how many fixtures you use at once and... what and how much of it is in the water that the softener will be removing. Click the link in my signature to learn how to do it. All you need is a minimum water pressure of 20-25 psi so the unit can regenerate properly.
Here is a recent link asking about water testing.