Yes, it is backward compatible with MoCA 2.0 & 1.1.
The MoCA 2.5 device will only communicate with each MoCA device at the max speed of that MoCA device on the network. For example on a mixed MoCA network:
ECB6250(v2.5) > ECB/WCB6200(v2.0) = 1400 Mbps
ECB6250(v2.5) > ECB2500C(v1.1) = 270 Mbps
ECB6200(v2.0) > ECB2500C(v1.1) = 270 Mbps.
Note: This is the MoCA communications speed between MoCA adapters. This is not the MoCA adapter connectivity speed to the computer or device connected via Ethernet or Wireless to each MoCA adapter.