BuddyPress.org

My advice:

1. Don’t copy bp-default. It’s a very old theme and is being retired. Instead, model everything off of the bp-legacy files. They’re at /plugins/buddypress/bp-templates/bp-legacy/

When BuddyPress runs (1.7+) it is pulling in templates files and CSS and JS from this folder. You can replicate that folder structure to override any templates in there, like CSS, etc.

2. If you want to completely override the default CSS, you can remove the enqueuing of it entirely with this in your functions.php, so that you’re not loading your CSS in addition to BP default CSS.

wp_deregister_style( 'bp-child-css' );
wp_deregister_style( 'bp-parent-css' );

3. Sounds like a nice plan of attack. I wouldn’t recommend using a separate stylesheet for every single component, because you’re going to end up with a ton of stylesheets which will be bad for performance. But if it makes your life easier to code this way, you can always combine all of the styles at the end. Also I think you may find that a lot of stuff overlaps between components. Like all directories are basically the same CSS (Member directory, Groups directory, etc.), and member profiles and single groups are basically the same CSS. So it may get redundant to separate by component. I would separate by directory, single item (profile, group), and then get specific for different “inner” content types – like messages, activity, etc.