If we do have a short well screen, do we see a significant difference between low-flow and no-purge samples assuming there is a reasonable ambient flow rate?

The shorter the well screen gets, the less difference you should see.  In studies that have been done by the USGS and others (the USGS being the inventors or the developers of the passive diffusion bag), they see significant changes in concentration for some parameters using the no-purge devices at different positions within a screen.  For example, in one study that was published in 2000 over a distance as little as seven feet they saw concentration differences plus and minus 150%.  But the shorter the well screen gets theoretically, the less difference you should see between no-purge samples and purge samples because in essence the water that you are sampling should really be the same.