A little history might be in order.
Slurry stones were "invented" by Japanese sword polishers long before there was sandpaper or diamond plates. They were used for de-glazing sharpening stones, knocking down high spots, and producing a particular type of finish. The coarser stones give rise to a hazy finish (kasumi), as seen in many Shigefusa knives.
At the polishing level, the ideal was tomo-nagura, or "same-stone" slurry. The sword polisher would use a broken-off piece of the same stone he was finishing with to produce the slurry. Finishing stones are very hard and fine-grained, and the result was a mirror finish.
To me it doesn't make a ton of sense to use a soft, coarse nagura on a hard, fine finisher. The polishing effect will be limited to whatever the grain of the nagura is, not the finish stone. Far better to use a tomo-nagura, or, in the absence of that, a diamond plate to raise a slurry from the finishing stone itself. There can be issues of diamonds coming off the plate, but in the rare event that happens, you can feel them with your fingers and get rid of them.
Coticule users have always understood this concept. Many coticules are sold with the equivalent of tomo-nagura slurry stones, that is, pieces of the same coticule or at least the same *color* coticule (blue or yellow) to be used for raising a slurry.
These are just my opinions, of course, and if a Botan is working for you, enjoy it!