How to show only content created by the user in the find content (administrative) page
I've found that it's better to leave the admin areas to the admin user alone, and to build custom Views for "manager/editor" users.  This gives you more control over the presentation and lets you easily add in additional information and links that aren't normally available in the admin screens.  Not only that, you get easy access control to the listings.

If you really must do it the way you're talking about, there's an article on the drupal.org site discussing the theme hook for that screen, which I believe you could override and then filter based on ownership of the nodes in the node listing:

http://drupal.org/node/108986