The scenario is like this:
If I raise my right arm, the kinect detects it and the as3Glue library sends a signal to the Xbee that will dispatch it wirelessly to all Arduinos, each one reacting in a way. Turning a light, turning a motor, etc, etc.
It is my first time dealing with Arduino. Can you guys give me tips about how to accomplish that? Should I start to try to hack the Firmata to react differently to the signal? I would like to avoid that, since I am not that good with Arduino language.
Arduinos are PROGRAMMABLE! It doesn't matter that they all receive the same signal if they're programmed to respond to it in different ways.
Using your example, if you raise your right arm one can turn a red light on, another can turn a blue light off and a third can flash a green light on and off. If you raise your left arm then the first one can turn the red light off, the second can turn the blue light on and the third can make the green light steady. The signals to each arduino will be the same, what each one does with the signal will be different.