I'm not an expert in filming, so I don't know if it's possible to have a longer shutterspeed than the amount of frames per second. Other canon cameras don't do that, at least.
After a quick test, I can't notice any brightness difference between 1/24 and 1/31. But something is happening, since I get some flickering when choosing under 1/31th second. The flicker is something like fluorescent bulbs, even without being close to fluorescent light.
Update: This is wrong, no flickering with shutter speeds lower tan 1/31 second :/ it might have been a fluorescent bulb...