May I use both USB-serial and tx/rx pins at the same time?
The usb port and the tx/rx pins are one and the same thing. You can't use one thing for two jobs at the same time. So you will either have to implement some kind of multiplexing system (switches) or come up with a different way of doing things, maybe using the SoftwareSerial library or the Wire Library or investing in a better board with more than one serial port.