In your scenario you would need 250 user CALs for Server 2012. These 250 User CALs can be used for any prior versions of windows server so you wouldn't need to also obtain CALs specific to Server 2008 and the Server 2008R2 CALs would be superseded by the Server 2012 CALs and be rendered obsolete.
Considering this, you can use downgrade rights for the newer 2012 server and install Server 2008 R2 so as to avoid the CAL requirements for Server 2012. Your Server 2008 R2 CALs already cover you for the Server 2008 Standard server. There is not a separate CAL for Enterprise or Standard.
You can read more about this here