"Is the XBee module compatible with other ZigBee and 802.15.4 devices?"
XBee radios are 802.15.4 and ZigBee network compatible and can reside in and around existing 802.15.4 and ZigBee networks. By default, the XBee contains some MaxStream proprietary information appended to the data. In this state, it will not be compatible, or possible to communicate with non-MaxStream devices. The XBee modem adds an extra header to the data portion of the 802.15.4 packet. This breaks compatibility with other vendors.
It is however certainly possible to communicate with specific vendor based systems using the XBee's Transparent Mode where all 802.15.4 data is sent as created - no additional information is appended, making it possible to match other applications.
The ATMM (MAC Mode) parameter has been added to allow users to disable/enables the use of a MaxStream header in the 802.15.4 RF packet. These new modes are MaxStream default mode (uses extra header), 802.15.4 no acknowledgements, and 802.15.4 with acknowledgements. When enabled (Mode 0, MM = 0) duplicate packet detection is enabled as well as certain AT commands. Modes 1 and 2 are strict 802.15.4 modes.