The gas system is irrelevant in regards to the LENGTH of the buffer you need.  Buffers come in two lengths: "rifle" (long) and "carbine" (short), and these correspond to the length of the buffer tube used.

A standard fixed stock of either A1 or A2 length uses a "rifle" (long) buffer tube, and so requires a "rifle" buffer and spring.

A real collaspable carbine stock uses a "carbine" length tube, and therefore a "carbine" length buffer and spring.

The confusion comes in because some of the fake collaspable stocks use rifle-length  tubes, and some of the new, short fixed stocks use carbine-length tubes.  Regardless, the buffer and spring must match the length of the tube.

Now, if you're using the carbine buffer system, and you're ALSO using a short "carbine" gas system, you may have better cycling with a heavier carbine buffer (or "H-Buffer").  The H-Buffer is probably not needed at all with a mid-length gas system, and certainly not with a full-length gas system, and would just add unnecessary weight.  The rifle-length buffer system is already fine even with a short gas system, due to its additional weight and length.

-Troy