Hello,
In reference to the Databases used for Replication; if the SEPM A is carrying SQL 2005 and SEPM B is carrying SQL 2008, still it should work.
For replication we use matadata that is data that is taken out from the database in neutral format and transfered to the other database.
NOTE: When adding a SEPM for failover and load balancing, it must be installed using the same version of SEPM installed on the first server. Installing a different version may appear to work, but the SEPM service will fail to start on the new SEPM.
However, in reference to the Failover / Loadbalancing, it's the configuration under MSL's of SEPM.