Best practice either create multiple databases or multiple users

As @ik_zelf mentioned, your PATH variable will need to include the "bin" directory of your Oracle software install. This will allow you to execute commonly-needed commands/utilities like "sqlplus" and "lsnrctl" without changing directories. This is typically $ORACLE_HOME/bin

Your TNS_ADMIN variable needs to be set in a similar matter, pointing to the location of your tnsnames.ora, listener.ora, etc. By default, SQL Plus checks this directory (defined by TNS_ADMIN) for tnsnames.ora. This is typically $ORACLE_HOME/network/admin

If you want to connect to your database via TNS, your tnsnames.ora file (assuming a ORACLE_SID of "myDatabase", a host of "myHostname" and the default port) would look something like this:

myDatabase = 
    (DESCRIPTION = 
        (ADDRESS = (PROTOCOL = TCP)(HOST = myHostname)(PORT = 1521))
        (CONNECT_DATA = (SERVICE_NAME = myDatabase)))

This would allow you to connect locally simply by doing the following from a command line:

sqlplus username/password@myDatabase

Of course, as SQL Plus looks for the tnsnames.ora file in the location defined by $TNS_ADMIN, this will only function locally. To access this database via TNS (Transparent Network Substrate) from a remote machine, you would have to install the Oracle client and ensure that the TNS entry for myDatabase (as described above) is included in the remote machine's tnsnames.ora file as well.

Some might say depends how big the databases are; I think regardless of the size, accessing the databases directly instead of having a "local" mySQL database to query would be best, for the following reasons:

• No need for disk/memory/maintenance of the mySQL database
• If the updates to the various databases happen every few seconds, and the users may only use the web page every few minutes/hours/days, you are executing a lot of queries that are not needed most of the time, which in turn places additional load on the six databases. In other words, I think the client doesn't realize the impact of having a replication scheme. Even if just a few tables from each database is involved, the synchronization code running every few seconds would be much worse than occasional specific queries (occasional being something running every few minutes/hours).
• The code to replicate data from 6 separate databases would be quite complex to do correctly.