t equilibrium setting in SWV measurements

The simplest way is to just use "set_e <potential>" to set the required potential, followed by a "wait <time>" command. What PSTrace does is slightly more complicated but it is unlikely to make a difference.

Can I set 500mV as a required potential every time? 

If 500 mV is within the range you have provided with the "set_range_minmax da <min> <max>", yes this is fine. If you have no used the "set_range_minmax" it will also be fine in this case, you will be able to apply +/- 1100 mV.

If 500 mV is within the range you have provided with the "set_range_minmax da <min> <max>", yes this is fine. If you have no used the "set_range_minmax" it will also be fine in this case, you will be able to apply +/- 1100 mV.