Quote.--You've lost me there Jon - where did I say the nipple had to be cut (i.e. which post)?
Could be my mistake. I read those blue dotted lines as cuts.-
I'm not sure about which MC to use but to fit a standard coupling I guess the two pipes to be joined should have the same diameter.
However if one pipe was plastic and the other copper as long as the outside diameter was the same then the coupling would be OK on both. ( Compression coupling. )