Be careful to follow the following steps religiously:
- Start or resume a campaign from the main screen.
- Go back to the main screen and select a single (non campaign) mission.
- Choose any of the single missions offered.
- Enter the briefing, click armament, and select the largest Herc there.
- Go to the main menu and resume the campaign
- Play the first assigned mission, then save your game.
- Load the save games
You now have all the Hercs with all the best weapons. Enjoy!