If you are providing a pump to your client who, in turn, is designing (and will be using?) the system, then it is in his/her best interest that the suction line be supplied in the correct dimension in order to ensure that the pump will operate to his satisfaction. Sadly, the customer is not always right.

You mention that there is a 20m suction head available. You may wish to consider if that was calculated with a 150 or 250 NB pipe size. Also, is it adequate for the pump operation?