S&W MP-15 interchangeable parts

[Basshoe]

Can someone clarify something for me? I have been told that I can use any AR-15 parts on my weapon. I have also been told me that I can only use M-4 parts. It seems to be a very confusing issue and would appreciate any clarification on it. I am interested in a quad raid replacement for my hand grips in order to add some tactical gear. Thanks for your support.