Yes, you can simultaneously connect external power supply and USB. As explained in one of the answers, that you linked, the Arduino chooses it's power input through the supplied voltage on Vin/barrel jack. Vin has no direct connection to the VUSB, so the USB port will not get any voltage from the external supply, thus it does not get damaged.
BUT: Your ...
Garbage values on output is mostly due to the baudrate.
Try changing the baudrate on the output terminal to the baudrate in the code.
Try checking the output by changing different baudrates on the terminal.
This must work for you :)
There are some good books available to help. There is the Raspberry Pi cookbook and the Arduino cookbook. These will take you from the very beginning to the point where you can make your own designs and they will work.