A BEC motor controller should be fine. Just make sure you have a common ground.
Indeed, the 4053 is an analog multiplexer, but it should work fine with digital signals as well (PWM servo signals). Digital is just analog to the extremes (really high, or really low).
The idea of using a 4053, is that you could switch between manual and auto control mode (you would switch which controller the servos are connected to). A 4053 is basically an electric 3PDT switch.
The servo controller I made and gave you a link to, is for controlling servos, much like the NXTServo from mindsensors. If you already have their controller, it would be pointless to make your own.
If you want to make the controller on a solder-less breadboard, you need:
Breadboard connection to NXT sensor port. I suggest you use an NXT female socket breakout
Breadboard connection to a computer RS232 port (only Gnd, RX and TX)
330 ohm resistors (one for each servo output, so 6 if you use all outputs)
header pins (3 per servo)
5-6v power supply (or BEC)
No need for a starter pack. You also don't need an IC socket if you are using a solder-less breadboard.
If you want me to, I can draw a block diagram of what I envision.
Edit 2: I didn't show how to control the 4053 (switch it), but it just requires a high vs. low logic signal.