The # 6 can only be used on a single ground rod if there isn't the ability to do one the other methods of GE.it is my understanding that you have to use a GEC sized off of 250.66 if you can install more than one. So if you use the ground rod as a supplemental ground it needs to be -example a#4 for 200a service.
I do not think this is correct. On what code section do you base this assumption on?
This section is pretty clear to me.
250.66 Size of Alternating-Current Grounding Electrode
Conductor. The size of the grounding electrode conductor
at the service, at each building or structure where
supplied by a feeder(s) or branch circuit(s), or at a separately
derived system of a grounded or ungrounded ac system
shall not be less than given in Table 250.66, except as
permitted in 250.66(A) through (C).
(A) Connections to Rod, Pipe, or Plate Electrodes.
Where the grounding electrode conductor is connected to
rod, pipe, or plate electrodes as permitted in 250.52(A)(5)
or (A)(7), that portion of the conductor that is the sole connection
to the grounding electrode shall not be required to be
larger than 6 AWG copper wire or 4 AWG aluminum wire.