This slug might be the Milax gagates, or something else. My orginal tentative ID was Tandonia budapestensis, but it does quite fit the typical characteristics of that species.

It was observed along Ellon Road, Aberdeen, Scotland, at night. My guess it was about 50 mm (2 in) long.

Thanks to invertzoo for all suggestions! Sorry, I did not pay close attention to the details of each slug, so I can't answer your questions.

I was quite impressed by the diversity of slugs and land snails in a short stretch of sidewalk!