I recently purchased a .308 AR matched set from aero precision. To be Bay State compliant can I put a collapseable stock and "pin" it and what extension tube can I run if not?
No one can tell you to be Mass compliant, you can put a collapsible stock on and then pin it. However, this has become the most accustomed practice WRT to the MA AWB.
Depending on the stock, you can run a Carbine buffer tube or a Rifle buffer tube. Many of the 308 LR parts are backwards compatible - just need to do your homework on which are and are not.
I have a mega maten set coming for a 308 build. I ordered a Lancer A1 length carbon fiber stock and a rifle buffer. I have come to prefer fixed stocks vs pinning adjustable, and I am looking to minimize weight. The magpul PRS stock weighs a shit ton. You should be able to use either a rifle or carbine buffer tube just get the appropriate 308 buffer and spring. Stick with mil spec tubes and buttstocks.
I like the Magpul Fixed Carbine stocks over pinning, if you don't want/need a A1/A2 length of pull.
For a 308 build, also consider the longer A5 length tube from Vltor or BCM (or Armalite's ar10 tube). It still allows for use of a carbine stock (of whatever variety), but is generally softer shooting, all else being equal.