Any combination of extensions work, as long as at least one is installed.
Suggestion 1: Do not allow (the button) to remove the
db.php driver drop-in if mysql extension is not loaded, as this "suicide". Not even on plugin deactivation. This suggestion is only relevant for the plugin as a tool.
Suggestion 2: For PHP 5.3 the mysql extension should have priority, then pdo, mysqli last. For PHP 5.2 the mysql extension should have priority, then mysql, pdo last.
Suggestion 3: Allow for a
wp-config.php constant to set driver (priority) as a string or serialized array:
define ('WP_DB_DRIVER', serialize(array('mysqli', 'mysql', 'pdo')));
Will get back to these issues on trac.