Sil83:
According to what is reported here https://www.arduino.cc/en/Reference/SoftwareSerial, since the microcontroller Pololu A-Star 32U4 Mini SV is based on ATMega32u4 as Leonardo, apparently I cannot use pins 18 (RX) and 19 (TX). Am i right?
You are right. The page you've linked gives you a list of the pins that you CAN use for software serial.