Time Lapse fps

Yes and no.
You can edit to the nearest 1 frame per second if you go to iMovie/Preferences and check "Display Time as hh:mm:ss:frames". With this unchecked, the nearest you can edit is to the 10th of a second.

Now, select a photo and open the clip inspector. Set the Duration to 5 frames. Apply to all photos. This is the shortest iMovie will let you go.

However, there is a hack that will let you get your 5 frame photo down to a single frame if you want.
Set the Project Thumbnail Slider to 1/2 second. On a photo, select a single frame by dragging your mouse. You should have a yellow border surrounding about 1/5th the length of the photo.
Then command-c (copy), command-v (paste). This pastes a single frame of that photo into your project. Now you can delete the 5 frame photo. So YES, iMovie can produce a sequence of single frame photos.

So the real answer is... NO
iMovie is not the right tool for this. It would be very tedious to do this for thousands of frames.

So what are the right tools? Here are several to look into.
*Stop Motion and Time Lapse*
QuickTIme Pro from Apple
iStopMotion2
Dragon Stop Motion
FrameByFrame

You can also try Gawker. Gawker records from any camera (video camera) that can run real time connected to a computer and turns the result into a time lapse video. However, you may not have the best of luck importing into iMovie.