To merge databases, first make a backup of your databases.  From the menu bar select File -> Backup Database.  Once the backups are complete then either create a new blank database or open the one that you want to be the master database.  Then you can restore the database that you want to merge it with the existing master database.

From the menu bar select File -> Restore Database.  From the popup window select the appropriate backup and click the "Next" button. Then select the "Restore Specific Profiles (Merges selected into the existing database)" option.  Click on the "Select All" button and set the other options as you see fit then click on the "OK" button to merge the databases.

To separate types of media (i.e. DVD and Blu-ray) you can create custom categories (one for DVD and one for Blu-ray).  From the menu bar select Tools -> Edit Collection Categories.  Once the categories have been created you can move the profiles to the appropriate custom category.