On most Macs, you just need the printer cable. On the Mac Plus, you may need to make one, as I do not know if a null modem DE9 to DE9 Mac serial cable was ever built. (I don't know what the printer end of things looked like on, say, the Imagewriter 1 or Imagewriter LQ.
I'm not sure what you mean by system connector.
I'm guessing he means the serial adapter to the 3-pin cables, or serial to PhoneNet adapters.
You
can build a 2-node LocalTalk network out of those system adapters, but you generally did not need them.
The only gotcha is that the Plus does not use DIN-8 connectors, it uses DE-9 connectors, so there may be a need to use two LocalTalk 3-pin or LocalTalk PhoneNet cables. The advantage there will be that you'll have the starting kit needed for a bigger network.
EDIT: here are a few graphical examples, this page suggests it should be possible ot use a "printer cable" but doesn't show a printer cable linking a 512 and a Plus or two Pluses.