I don't know much about them, but would African blockheads be suitable?
Mature Congo Tetras could live with Steatocranus Blockheads, but it would mean a compromise for both species, because the Blockheads come from the rapids around Malebo Pool whereas the tetras come from much more sedate waters (and their caudal fin does not have the clean fork shape). Setting up the tank somewhere inbetween would be mediocre at best.
A far better African tetra to put with Blockheads are Long-fin Tetras, Brycinus longipinnis, who do enjoy a good current.