Q: How do I combine all users into one? iTunes, documents, pictures?

Depends on how much data there is in total. You could just save all the data into the 'Shared' folder on the hard disk by going into each account and making copies or moving the data you want to keep. When that's done, switch to the account you want to move it to and copy it all in from the 'Shared' folder.

If there's too much for your HD, copy it all off to an external USB hard disk, delete all user accounts apart fromthe one you're going to use, then copy it from the external back into your admin user.