Most houses of less than 2000 sq feet use a 3/4" supply line. My concern would be that if the filters are not serviced adequately, that they will restrict flow which will prevent thorough backwashing of the softener.
A lot of people throw in a 5 or even a 20 micron filter and then wonder why their flow rate went down? A red blood cell is about 5 microns in diameter. An average human hair is about 75 microns in diameter. Realizing this, the problem with cartridge filters is evident. Disposable whole house filters tend to cause more problems than they solve.
40/60 is fine water pressure. If your pump can maintain this at the softener while it is backwashing, you won't have a problem.