Quote Originally Posted by Pence View Post
*Completing the objective with a party containing representatives from all three Grand Companies will earn you greater rewards.
This is correct, and has been tested. If you are lacking representation of any 1 or 2 Grand companies, you will still only be awarded 5000 seals. the extra 1000 is a bonus for inclusion.

Much like defeating Lesser Ifrit and Lesser Garuda with only one represented GC awards its own bonus.