RG0 is special and is always associated with the Control Plane - whether Node 0 or Node 1 RE is Primary (Master) or Secondary. Sync will happen across the Fabric Link. Other RG's are associated with RETHs which are associated with physical interface(s). A reth interface is a virtual interface. It will be active on one of the two nodes only and it has the ability to move/failover to the other node.
Only one Node will have any specific RETH as primary; other side will always be in secondary. See page 19 of 3rd URL - cut and paste does not work well.
SRX Cluster can not function properly with out both RG0 and some other RG. I 'think' this is the question you are asking?? See pages 4-9 of the HA deployment guide for all details.