Can I use Rx/Tx aka Pin0 Pin1 on the arduino mega to communicate to Mything? Sending to mything with arduino TX->mything RX should work, I think. But how about the arduino rx? Is the usb/ttl converter leaving it floating while there is no signal recieved from usb, so that I can splice mything TX into it?
Yes, you can use the RX/TX pins for communication. You could use any other pair of pins, though, using NewSoftSerial to simulate a serial port, so as to not interfere with the hardware serial port, leaving it free for debug use.
It buffers and modifies the bi-directional communication .
You don't have all that much space to buffer data.
What is mything going to be doing that the Arduino can't do?