[Just bought a vq35de]

Untrue... Well the guys in the Nissan service dept say that with the VIN on the ECU, they can program a BCM to read any key. (I believe this because what if you had a 350Z with a Bad BCM? they'd just order the BCM, reprogram it with their ConsultIII, and vroom) Trick is that you have to have an ECU that matches the VIN. So try to get the VIN from your donor car, and write it on the ECU. It's a good thing to get to know the Nissan dealer service manager, and lead techs.

 

I have first hand experience with this. The BCM does not need to be from the same car. I'm not sure whether it's the BCM that they reprogram (in the case of a used BCM like the one I bought) or the chip in the key, or both. Nonetheless, I know for sure that you can use a used BCM from another car. You will need Nissan to do the programming to allow the car to start also.

[Just bought a vq35de]

If you want to use all of the stock parts, they do not need to be from the same car. The immobilizer is part of the BCM so you will need to have Nissan program the keys you end up using to work with the BCM that you buy.