Any of the mid-sized American cyprinids will work with comets, commons, and the less-delicate fancy varieties. I have kept goldies with adult Pimephales (fathead minnows/rosy reds), Nocomis (river chubs), Notemigonus (golden shiners), and Semotilus (creek chubs). These minnows are rambunctious fish and assertive feeders, so should probably not be kept with bubble eyes or other delicate fancies.
I also had a similar-sized warmouth (Lepomis gulosus) and comet together for a while. I saw no issues, but warmouth are shy fish and probably don't appreciate the constant activity and inquisitive nature of goldfish.
I also had a similar-sized warmouth (Lepomis gulosus) and comet together for a while. I saw no issues, but warmouth are shy fish and probably don't appreciate the constant activity and inquisitive nature of goldfish.