Is it possible to define a different Auth config for a bundle ? application/config/auth.php is default for bundles as well but I need to use different auth modules for different bundles of the system.
Also, creating a new one on the BUNDLE/config/ folder doesn't affect anything. So, it loads the default one in any case.
The short answer is that you cannot... Neither Auth nor Config were designed to be extended per bundle.
The long answer is that you could use your bundle's start.php file to override each config setting, for example
// bundles/my-bundle/start.php
Config::set('auth.driver', 'fluent');
Config::set('auth.table', 'my_users');