Depends what you want it to look like.
Let's assume that you shoot 18fps on your super-8 camera and have it transferred so that there is one super-8 frame in each video frame. The video file will probably be at 25fps so if you play it conventionally, naturally, everything will seem speeded up.
As you say, you have two choices: get the talent to perform more slowly by slowing the track down to take 25/18=1.389 times as long as it usually would. Personally I think this might risk looking a bit goofy, depending on the style of music. It's much more common to have people perform more quickly and shoot at a higher-than-normal frame rate. You'd be having people perform more slowly, and shooting them at a lower-than-normal frame rate. Your call, depends what you want. Shoot tests.
The other way is to import the 25 fps file into your edit software and interpret it as 18fps. Assuming you're editing on a 25fps timeline, this means that the NLE will (unless you tell it to use advanced tricks like optical flow interpolation, which I wouldn't) duplicate occasional frames to stretch out the 18fps material. This will be lumpy and uneven, since 18 doesn't go into 25, but it might be acceptable as a sort of vintage super-8 look.
And super-8 is probably going to look a bit vintage no matter what you do, unless it's shot with absolutely exquisite care on the very best equipment.
If you don't like either of those options, I would strongly suggest you find a camera that can shoot at 25, or at least at 24 and make it a 24-frame production. Super-8 cameras aren't exactly expensive, although the most desirable models have held their value amazingly well considering.