Yes, the S1 and the S2C are compatible when running the 802.15.4 protocol. The legacy modules S2 and S2B are not compatible with the S1 but are still Zigbee compatible with the S2C.
The legacy S2 modules only support Zigbee and Smart Energy protocol for communication. The XBee S1 module supports IEEE 802.15.4 and DigiMesh protocol.
The XBee S2C module now supports IEEE 802.15.4, DigiMesh, and Zigbee protocol.
Last updated: Sep 20, 2017