Can I add any existing username and password with the required privileges as control user? If so, do I have to do anything other than updating the following fields in config.inc.php file?

$cfg['Servers'][$i]['controluser'] = 'existing username mentioned above';

$cfg['Servers'][$i]['controlpass'] = 'existing password of the above username';

I am getting message like "Connection for controluser as defined in your configuration failed" in my phpmyadmin page. even after I did above things.

Also , do I have to set the control user separately for each database I would like to configure for additional features?