For fixed stocks (A1, A2, Cav Arms, Sully stock), you can use the same fixed stock buffer tube and buffer for any of them. The A2 has a little spacer that goes in there.
Collapsable stocks are different. Not only do they attach to the receiver with a locknut, but the tube itself is different for every type of stock (M4 type, old style two position, four position, etc). There are also differences with some being milspec, some not (different diameter tubes). Carbine stocks use a smaller lighter buffer. I run a heavier 9mm buffer on my carbines (weighs the same as a fixed stock buffer), but then again, I run Midlength gas systems so it's not as violent.
I'm not sure if the springs are different.
Now as far as barrel length, it doesn't matter. You can run a collapsible carbine stock on a 20" and a fixed stock on a carbine. No problems. I do like to get the heavier carbine buffers though, just to keep everything same-same.