Why? Mac Pro has intergrate AirPort Extreme...
Sure. Some of the newer Macbook Pro's don't use a PCI-E Airport card though and the Mac Pro may not have enough antennas for all of the card's ports. But it'd work just fine.
Not on the 2009 (4,1) model. It was optional. Until the 2010, wifi was optional on all Mac Pros.
I'm pretty sure it would work, I did it the other way around. (Bought a MP card and installed it in a MBP).
It'll work, I've done it myself, with my sisters dead MacBook Pro, just make sure you discharge it from electrical residual from the battery, otherwise you risk the logicboard to have a short circuit.
But 100% it'll work