On the box of shells will it tell you to use IC or Mod? I have both so I guess I could try both.

No, it doesn't matter. Most manufacturers will tell you to shoot them out of the most open choke you can use, which would be cylinder bore. Most guns do not have this choke anymore, I/C, which stands for Improved Cylinder is about as close as you will get. Modified is one step tighter.

You can shoot a rifled slug through just about any choke except for extra full and turkey chokes. The fins on the slug are not meant to create spin on the slug, they are there to crush as the slug goes through the choke.

The problem is not all slugs are made the same. As a matter of fact not all barrels are made the same. Many will vary several thousandths of an inch from manufacturer to manufacturer, both barrels and slugs. So by trying several different combinations you are finding the the right choke, barrel and slug that fits the best. Harmonics are also a big part in it.

Paul