Maybe some Rosy Red Minnows (Pimephales promelas)? Maybe a couple of weather loaches (Misgurnus anguillicaudatus)? Both of those will get larger than the mouth of the goldfish. Just not sure whether you would be able to squeeze in a small group of weather loaches.
Also, the rosy red minnows are usually sold as feeders, which could be risky as they might have diseases considering the way that they are usually kept in LFS. Although, I have kept them with 2 ryukins, a fantail and an oranda.