Disclaimer: I am a cherry to mpcnc, as I am still in the build process myself. Hopefully have it working this weekend.
The different couplers and nut locks are if you are building with US or EU specs. 5mm to 5/16ths is US. 11mm nut locks for US.
Those are what I used and my center piece went together with no problems.
Hope that helps
Reply To: Multiple Nut Lock and Pineapple Coupler files.