I have a 20n A2 buttstock and want to put a carbine style magpul on it.  Will a rifle buffer work in a carbine stock?

You will most likely have to go to an adjustable stock buffer tube, the fixed stock uses a different buffer tube than the adjustable stocks.  The Carbine buffer is also different than the rifle buffer.

http://www.gundigest.com/guns/rifle-reviews-articles/gunsmithing-the-ar-3-tips-for-buffers-recoil-springs

No, and neither will the spring.

It's rifle-rifle-rifle or carbine-carbine-carbine (excluding Vltor A5).

So 20in barrel would need the rifle buffer right?  So I can't use a adjustable stock?  

I didn't say that, You can, but you will have to play a bit to find which combo your rifle likes, I have a 20" with an adjustable stock, but I had to play with buffers a bit to find the right combo to make it work correctly,but it can be done.

You can pretty much do anything you want with these guns, you just have to have the patience and be willing to experiment until you find a combo of parts that works.

You actually have to replace the whole buffer tube/spring kit along with the stock you want. The A2 buttstock run with a rifle length tube and spring. Adjustable stocks run with carbine tube and spring. Carbine tube/spring works fine on 18" or 20" barrel (mid-length or rifle length gas system, although I did have to tweak my buffer spring a bit on my 20").

A Magpul MOE rifle stock will fit with any other changes

MOE RIFLE

There is one adjustable stock that I know of that uses the rifle buffer and spring and that is the ACE M4 SOCOM LONG.  They make two lengths, short and long.  The short uses a CAR length spring and buffer, the long a rifle length spring and buffer.  The stock comes with its own buffer tube.
The length is adjustable on both the long and the short and IIRC, length is such that the long version is at its shortest about the same length where the short version is at its longest.  They are pricey, pretty heavy, but also a pretty darned good stock, and I don't claim to know a quarter of what is out there, but it is the only quickly adjustable stock that I know of that uses a Rifle buffer and spring.  The Magpul PRS is also an adjustable length rifle stock but it is very heavy and not exactly quickly adjustable.  It is more at home on a precision rifle than the type of rifle you normally think of wearing an adjustable stock.