Fixed Buttstock vs. Adjustable

[copterdr]

I am kind of new to the AR building world and was just curious if I could use the same buffer spring of one of my old M4 carbines for my new fixed buttstock on my Grendel. I do know that the buffer assy is different just not sure on the length of the spring. Also I haven't seen very many critiques on the Jard 2 stage adjustable trigger, and was just looking for some feedback. Thanks in advance!!!!!