The largest ZigBee install we have done has 2 XP-8's, 4 ZRP-6's; each with a ZM-24 and a T3-Vz. There are also 2 RK3's and 9 RKM-1's. It has been fun to set up but seems to be stable. Read the ZigBee installation manuals as they are of great value. It is not like a 433MHz model in that the processors and antennas can be loaded and powered up at any time. The first XP-8 in the system acts as the co-ordinator and the other processors with ZM-24's connected are repeaters. We also needed 2 stand alone ZM-24's to complete the network based on the ZigBee test driver. There is still some programming to complete and refinement of the GUI but it does seem to work well.

My personal system relies on 2-way as well with an XP-8 and T3-V with drivers for an Escient FP-1. Kaleidescape and Aprilaire 8870 and this has not given me any concern. I have an Apple Airport Express near the antenna and 2 WiFi networks n my house with no interference issues. AndyM is dead right suggesting that you test the network. Stand-alone ZM-24's solved reliability issues for us.

I do recommend that you play with the ZigBee products before you put them in field.