No specific interaction is documented for Radish and Rhubarb in the sourced chart. The compiled extension-backed companion table has no good or bad verdict for this exact pair.
That makes this a normal garden-planning decision. Put the crops together only if their mature size, watering needs, sun exposure, and harvest timing work in your bed. Keep family rotation in mind if they are related crops.
No companion-planting benefit or incompatibility is claimed here because no source row was found for this exact pair.
Spacing tip: Use the normal spacing for the larger crop, then tuck the smaller crop where it will still get light and water.
Sources
No extension-backed source row was found for this exact pair.