The 12.52 Policy Server is backward compatible with the 12.x agent and agent option pack. Regarding the FSSUI, once you have upgraded the Policy Server, Policy Store, and AdminUI do not use the FSS UI as you could corrupt your policy store. All of the functionality that was in the 12.x FSSUI has been incorporated into the 12.52 AdminUI.
Here is a link to the 12.52-SP1 SiteMinder Upgrade Guide: https://docops.ca.com/ca-single-sign-on-12-52-sp1/en/upgrading
Here is a link to the 12.52-SP1 Bookshelf: https://docops.ca.com/ca-single-sign-on-12-52-sp1/en