JUST MY OPINION

If you're using slurry, I think it should always be a powdered version of the exact stone you're using, which means the nagura stone is either

1. a piece broken off from your larger stone, or
2. diamond

There are dozens of different kinds of stones out there being sold as slurry or nagura stones. Unless you've been polishing Japanese swords for 20 years, I don't see how you could intelligently pick one to use with your particular finisher, much less a progression.

JUST MY OPINION