1) Yes SSO 5.5 is backward compatible
2) only need to upgrade SSO 5.5 and web client 5.5
Everything is here VMware KB: Supported vCenter Single Sign-On 5.5 configuration and interoperability with vSphere 5.1
Customers running vSphere 5.1
- If you are on vSphere 5.1 and have experienced either of the above two items above and unable to upgrade the entire solution to vSphere 5.5, upgrading just the vCenter Single SIgn-On and vSphere Web Client only to version 5.5 is the next best offering to eliminate these issues, and fully supported by VMware.