Can CME & SRST be installed on the same router. ( don't think so), If not what's the technical reason for that?
Yes it can.
However it has some restrictions...
Restrictions for SRST Fallback Support
â¢The call-manager-fallback command, which is used to configure Cisco Unified SRST, cannot be used on a router that is configured for Cisco Unified CME.
â¢The telephony-service setup command and auto assign command must not be enabled on a Cisco Unified CME router configured for SRST fallback mode. If you used the telephony-service setup command before configuring the router for SRST fallback support, you must remove any unwanted ephone directory numbers created by the setup process.
â¢The number of phones that fall back to a Cisco Unified CME router in SRST mode cannot exceed the maximum number of phones that is supported by the router. To find the maximum number of phones for a particular router and Cisco Unified CME version, see the appropriate Cisco CME Supported Firmware, Platforms, Memory, and Voice Products document at http://www.cisco.com/en/US/products/sw/voicesw/ps4625/products_device_support_tables_list.html.
Hvae a look at this document on how to configure srst fallback with CCME