For me the whole issue of frame rate is an interesting one, I know that 24 frames per second has been standard ever since the arrival of sound films in the 1920's. When using film most will film things at 24fps because that's how it's done professionally, but most of us don't ask the question why. Why is 24fps the standard? The standard speed could have been 20fps, 23 fps 26 fps etc. I wonder if 24 fps was chosen due to the infancy of the earlier sound recording devices? Maybe film needed to run fast enougth to produce decent quality of sound. I know that the faster the film runs it produces much smoother and more fluid and natural movement, some say that with a faster fps the grain is less visible, others say it adds to more realism. I have shot a lot of Super 8 at 24fps but I have also shot a lot at 18fps, the only difference I have noticed is that 24fps is better with handheld cameras whilst filming fast movement.
Slowing footage in NLE shouldn't be an issue as long as you've had a HD telecine, try and see. I have reently shot Super 16 at 18 fps and have had a 2k transfer and the slowed it down in NLE to 18fps and then synced my footage. The difference of my 18fps footage when compared to my 24fps footage isn't an issue for me as I hardley notice it and doubt that many would either.