Hi,
When a tape drive device is created a default media pool can be selected. After this, when that device is used for a backup the media will be used from the default media pool for the drive. The 'No media pool selected' option in the backup specification means that it's going to use the drive-defined pool but it gives you the opportunity to over-ride this if needed.
The 'Load balancing' is the amount of drives the backup specification will use. By default, whenever a new job is created it will be load balanced and you can choose how many (maximum and minimum) drives will be used for the backup.
The media pools do not define how load balancing works.
Hope this helps,
John