I believe you're limited to 60i, 30p, or 24p with the US model, 50i & 25p with the Euro one. Shutter speed can be adjusted independently of frame rate. There is no slow motion mode on the Canon as far as I'm aware.
You can pull off a 50% slow-mo by filming interlaced at either 60i or 50i (depending on if you are using the US or European version) and then deinterlace and convert to 30 / 25 fps. But as you are wanting a shot of a dart in flight this is unlikely to be sufficient slow down to produce the effect you are looking for.
FCP has optical flow retiming which can allow some measure of retiming, but don't believe there is anything in FCE that would help (anyone chime in and correct me if I'm wrong, don't use FCE personally).