It is possible to use Site Minder to protect Identity Manager without fully integrating the two products. It is however, not recommended due to a potential security hole. Mainly you will need to ensure that IDM framework authentication is disabled, that SM is using same directory for authentication and authorization and that you disable IDM's check of the header variables.
Identity Manager 12.5 and 12.6 do not require Site Minder. Identity Manager can run standalone.
Idnetity Manager integration offers a number of features. However, if not needed you might still want to protect Identity Manager with Site Minder and allow a single sign on with other protected apps.
In order to do that you will need to: