You can use master-master replication for client databases but make sure that you exclude the databases "mysql" and the ispconfig database (dbispconfig). If they would be in a mysql master/master replication the setup will fail as ispconfig manages the replication of mysql users and its own database content automatically.
The remote api is not related to replication. It is a API for external applications to connect to ISPConfig.
If you have to reconfigure the mysql port, you will have to edit the db_mysql.lib.php file in the server classes directory (/usr/local/ispconfig/server/lib/classes)on the slave servers.