Okay, here's my question. When I select a media file (mostly photos) and go to "open with", it shows about 20 choices. Is there a way I can customize this to show only the apps I want, or to at least delete some of them?
It gives you what it thinks you would most likely open it with. As far as your question I dont think that is really possible without deleting the programs that come up that you don't want to. I may be wrong however.
I've never found a way to edit that menu. Nowadays, I almost exclusively use LaunchBar. You just select the file you want to open, hold down the LaunchBar shortcut (control-space for me; I think it's command-space by default), and then when LB opens up hit the abbreviation for the app you want and the selected item gets opened.
I think the functionality is called "Instant Send" if you're looking for it in the LB promotional materials or documentation.