Re: list custom members
There are many functions in bp-core.php and many template tags in bp-core-templatetags.php that will retrieve various user data. Take a look in those two files.
You should not write any custom DB classes unless you are creating a custom plugin. Also, you should never modify core BP files.
If you are simply looking to list certain members, you can try my Featured Members Widget available in the WordPress plugin repository. You may have issues if you are running WPMU 2.8.1 as I have not yet updated my widget to use the new WP Widget API.