BuddyPress.org

It’s okay to do anything you want with the theme files, just as you would do in WordPress.

You don’t need the loader.php stuff if you don’t want to, however, it does add some extra functionality, such as cross theme CSS and the ability to selectively load styles.

You should also read this:

https://codex.buddypress.org/getting-started/using-the-buddypress-themes/