Now back to the OP's original question:
I have both the PC and DDMs Stage 1 Turbo.
Yes, you can use them both. They are independent systems.
Yes, they can get you into trouble REAL quick. I have learned to drive with an egg between my foot and the pedal.
Yes, I have mine turned down quite a bit too - just a couple settings higher than stock, but not any where close to extreme (I will have to see what the actual setting is next time I go out).
Worth the money is a personal question. I've got some really expensive stuff that was worth every penny and more. I've got cheap stuff that was just a waste.
Realize that the PC is just a foot multiplier. All it does is take the input from your foot and gas pedal and multiplies it to go to the ECM. So instead of you having to mash down hard you only have to push a little bit. If you do mash down to WOT, all you're going to do is spin your rear tire. That's why people say don't use it in the twistees coming out of corners but I have learned to control mine pretty easily.