I'm not sure what your question is. Are you asking if downtube clamp-style shifters can be mounded on the quill portion of a stem? It's possible, but the easiest solution is to have the smaller clamp diameter (22.2 or 22.0 mm). The downtube clamp will be larger (28.6mm, I think offhand), so will spin loosely when mounted on the stem.

Is that what you're experiencing? Why not bar end shifters on that mixte?