Below are a couple of good documents regarding configuring MGCP gateways with CCM. Yes, you can run MGCP and H.323 on the gateway at the same time to allow CCM and CME to coexist. If you have multiple ports for outgoing calls, configure some for MGCP to be controlled by CCM and leave some controlled by H.323 dial peers during your testing phase. Obviously, you will want this phase to be as short as possible as you are diminishing the number of available trunks.
Configuring Cisco CallManager with IOS MGCP Gateways (Analog FXO, FXS Ports)
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_tech_note09186a008009428e.shtml
How to Configure MGCP with Digital PRI and Cisco CallManager
http://www.cisco.com/en/US/tech/tk1077/technologies_configuration_example09186a00801ad22f.shtml
Hope this helps. If so, please rate the post.
Brandon