I think your main concerns with that kind of set up are predators in the area (Cats, Racoons, etc), shading during the hottest parts of the day, and the type of goldfish that you buy. Some of the fancier goldfish are more prone to disease due to inbreeding, just like many feeders are prone to disease from the same reason and oversytocking. Just check the fish daily to make sure that they eat and are swimming actively. Fish are meant to live in lakes and ponds, so the concern would be more about biomass per gallon. The more fish you have the less stable your set up.
My grlfriends feeder from the fair lived to be about 12" long and the size of a football.
I wish I could do that here in New York