If you use dissimilar stones for slurry as opposed to the hone and you use a harder slurry stone you will wear you hone and most of the slurry will be from the slurry stone which might defeat the purpose of using the hone in the first place. Usually the slurry stone is the same as the hone or is softer. Very often with Japanese Natural Stones the slurry stone (nagura) is softer-way softer, almost like chalk.