I have read in somewhere that I can use pin0 and pin 1 to send data to an external device
True.
with rs232
False. You can't send RS232 signals with a TTL chip. You can send TTL level data to a MAX232 chip, which can then output RS232 levels to the device.
You can't talk to two devices using the same set of pins. You can't read from the Serial Monitor and send to your external device on the same set of pins.