Your get('db.active') returns null because you haven't defined it, it is not in your db.yml.
If you specify your config files are yaml files, they all have to be, the framework doesn't merge config files of different types. With php files you get a lot of this default because there's a base db.php config file in /fuel/core/config which provides you the defaults. But there is no db.yml file, so no defaults to start with.