[sOLVED] Alright, project completed!
I realized this was a super old post: but it was the first thing I found from a google search with the hardware I was working with. I hope other people with this hardware setup can find it and shortcut the experimentation for predictable results.
What ended up working was the 18-55 kit lens Nikon DX AF-S Nikkor 18-55 1:3.5-5.6G II ED. That guy was zoomed in to the 55mm mark. I focused with the aperture all the way open, then dialed it down using the fobs on either side of the slide. I modified the shutter speed depending on the wide range of exposures that my folks gave me.
There was some cropping that happened as I wasn't able to mess with the bellows and focus that much. I had the focus all the way to one end with the lens, kept the bellows at minimum distance from lens to camera (essentially only using bellows to hold camera steady), then slowly moved camera/bellows/lens in and out until I got the focus as large a frame as I could. Now that I'm typing this, I think somehow fixing the camera and PS-6 to the same surface, and leaving the lens on the camera probably would've been a better setup with more focus options and probably no cropping in the end at all.
I didn't end up using the smaller bellows (accordion thing) that was on the PS-6 at all. It's got a round mechanism for grabbing on to the front of a camera lens, but it didn't fit the 18-55 lens cap area. I taped that PS-6 bellows to the front of the lens, and found zero noticeable difference; so I proceeded without it.
I think that pretty much covers everything. If I do this project again, I will probably fashion some mount using only the Slide Copying Adapter PS-6, a piece of plywood, and affixing the DSLR with lens to the plywood.
A video of how the stuff fits together
Final version overlaid on the slide so you can see the crop that happened.
Final version of digitized slide
Holding slide in front of the incandescent lamp, pic snapped with smart phone only meant as reference for the crop comparison
A comparison of the Nikon DSLR setup vs holding a slide in front of a lamp while snapping best pic I can with my Pixel 2XL. I'd say it was worth it!