802.15.4 is a Peer to peer, point to point, point to multi-point network. You need to use the Zigbee Mesh firmware for Routers.
To set a sleeping end device on Zigbee, simply use the router function set and enable sleep (SM).
No, 802.15.4 will not communicate with the upper Zigbee protocol.
Not at all. You just need to understand that End devices are generally low powered and sleeping. This makes it a little more difficult when developing a product as you need to keep track of the CTS lines on the module and know when it is awake.