Can you use less then the rated current on a High Side Smart Switch such as BTS555
Yes, you can use the switch for lower currents. When they list the load current as having a "minimum" of 128A what they mean is you are "guaranteed" that the device will be able to drive up to that minimum current and keep working (provided you follow the other specs for temperature/power/etc.). Some devices may perform better than that and safely drive more current, but they don't guarantee any higher than that value.