Hi Sam,
You don't need the CEM functionality for CA APM Manager for CA SSO (aka "CA Wily Manager for CA SiteMinder") as it just relies on the Introscope side of APM.
So yes you just install the EM(s) and CA APM Manager for CA SSO agent to connect to it.
Let us know if we can help further.
Thanks
Lynn
P.S. As an aside CEM functionality is built into the EM, plus you also need a Transaction Impact Monitor (TIM)