Are you in Durham, NC? Lots of cool local fish there. I suggest fliers or dollar sunfish. Dollars are a bit feisty, but your goldfish will be too large for them to intimidate. Flat bullheads would also be a good addition.
I suggest staying away from Myxocyprinus. It gets too big (up to 3 feet or more) and is too finicky. There are some members of its family (Catostomidae) local to you that would be more appropriate, such as white suckers (Catostomus commersoni), chubsuckers (Erimyzon sp.), and hogsuckers (Hypentelium sp.). These species' maximum sizes are 18-24", and most individuals stay smaller than that; if they get too big they can go in your pond. These species are a little more tricky to keep than goldfish or sunfish, but if you are interested I can point you to some care info.