If you have a good recipe you don't need the stearic acid you can depend on the oils to give you a hard bar of soap.
I use raw silk fibers, so I can't help you much there, but if the manufacture recommends that amount then I would suggest using that amount or less if you want on this particular item. (I wouldn't recommend going less on some things but I think on the silk it would be ok, but I would personally use what the manufacture recommends myself.)