Is temporarily reducing the water supply line an option when installing a water softener?
The lucklihood is that it will reduce the pressure throughout the house significantly, especially if more than one faucet is running at once.

It is also possible that the softener might not work correclty due to the reduced pressure, or that the output pressure will be even less than expected.

I would think that hard water for a couple extra days would outweigh soft water with little pressure - especially in the shower!