Hi -
Here are the SRND guides for CUCM 7.X and 6.X which explain the supported deployment methods for CUCM, Unity Connection and Unity Express.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/7x/vmessage.html#wp1086100
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/6x/uc6_1.html
There is a minimum version of Unity Express and IOS. Per the 6.1 guide, "The most likely deployment model to use Cisco Unity Express is the multisite WAN model with centralized call processing, where Cisco Unity Express provides distributed voicemail at the smaller remote offices and a central Cisco Unity system provides voicemail to the main campus and larger remote sites." You can also network Unity Connection and Unity Express subscribers via VPIM. Make sure you confirm the versions are compatible.
Regards, Ginger