Connect GPS module (WeeBee) to Arduino Uno
The article on the Arduino website means that you can't upload sketches from your PC to the Arduino or receive any sort of data, through the serial monitor or otherwise, from your PC, while the GPS module uses the RX pin. Any attempt to send data from PC to Uno in this condition would cause the Arduino to receive only corrupted data, since 2 devices (GPS module and PC) would be trying to send data at the same time, with the same line. This limitation is easily resolved by connecting the GPS module to a SoftwareSerial port instead; this keeps the HardwareSerial port (pins 0 &amp; 1) free for bidirectional PC-Arduino communication.
The datasheet says that the maximum power supply voltage is 5 V. However, the serial connections are 3.3 V logic, so yes, you will need logic level shifters. A simple voltage divider will serve. However, 'external decoupling' refers to circuitry used in decoupling/smoothing out any spikes or dips that may occur in the supply voltage. An untamed spike could damage the device, while a dip (perhaps from large current draw) could cause the device to stop working, if the voltage falls low enough to be considered under-voltage. A 10uF electrolytic capacitor, connected from Vcc to GND close to the GPS module, should be okay.