Can I use serial port and RFID reader on Arduino Pro Mini?
The ATmega168 has separate pins for I2C (PC4:5), SPI (PB2:5), and UART (PD0:1). There will be no conflict between the devices provided you use the MFRC522 via either SPI or I2C. Note that the Arduino Pro Mini is programmed via its UART, so you may need to disconnect the HC-06 when programming via the serial port.