I ordered one gosling from Metzer to be raised along with my ducks and seramas. For her shipment, i ordered 2 ducks to be mailed with but you only have to have a minimum of 2 birds total when it comes to ducks and geese. I only have one female Super African and she has never been a problem in the year that we've had her. She acts as a guardian a lot of times and immediately comes around when a foreign animal/person is in the yard. We have also witnessed on multiple occasions her pulling off aggressive roos and fighting ducks so they're great as a guardian and a peace keeper.
If you're just going to go with one gosling, make sure that you get a female. I'm sure that you already knew that but having just one male goose with no females, in a flock of ducks would just be asking for trouble. All in all, have one female goose with your ducks is a fantastic idea! Definitely no regrets with that decision. Just read up on the breeds of geese to help determine the right type for your flock. I went with the Super African because they continuously grow for 4 years, they give off an intimidating look yet and incredibly sweet(if you raise them right! VERY IMPORTANT!!!) and are known to protect their flocks.
I hope this helps you out on your decision making!