Q:

Can Arduino Leonardo use USB port and RX TX serial pins at the same time?

Can I use the TX/RX on pins 0, 1 (to a serial slave device) while also having the USB port dedicated PC communication? 

A:

32u4 is multi-serial uC. USB connection works on default serial communication which can be used via Serial.print("somevalue"). On the other hand if you want to use RX/TX on pins 0, 1 which are actually RXD1 and TXD1. So, if you use Serial1.print("somevalue"). So the answer is a yes. You can use USB and RX/TX (hardware serial) at the same time. 

