Hello,
The site https://supportforums.cisco.com/document/36746/uc-500-software-packs lists all supported model/firmware configurations.
The 9951 phone is supported since UC500 Software Pack 8.6 which includes a phone load version 9.2.2 Since you are running the latest release you are good to go.
If you enable the load within CCA and add the phone to your UC500 network (I needed to add the phone's mac manually within CCA) it should get up/downgraded to 9.2.2 and register with the UC500.
However, don't expect too much functionality. 89xx and 99xx phones are only supported via SIP (not SCCP) and are missing most of the nice features.
Edit: On a sidenote the UC500 series is EOL and the final hardware-support for the UC500-Series ends in early 2017.
Fabian