Schultz Pond material is very lightweight
While it is certainly cheaper than Flourite, the Schultz Aquatic Plantsing Soil is very light weight, it is probably OK to mix this in but I wouldn't try to use that exclusively. I've read that people have had trouble keeping plants in the soil using that. I've got some now, for use in the pond, but as the goldfish throw it out of the lillies I end up vacuuming it up and throwing it out. I may try to use some as a base in the next tank I set up, but not much.
The thing about the water disturbance is that if you add CO2 you want still surface to avoid the \co2 escaping and the level in the tank falling to ambient CO2 levels (3 to 5 ppm). If you don't add CO2, the plants will tend to use it up and you may want to have more surface distrubance, maybe even air pumps, to get the tank CO2 back up to ambient levels, as 5 ppm is certanly better than 0.
Now, as I have not read the Walstad book, I don't know where the soil planted aquarium is in this argument. So much CO2 comes from the bacteria in the filter, but much much more bacteria live in natural soil, so the soil itself would be the source of CO2 in the naturally planted tank. I suppose in that case, again, you'd want a still surface, but nice current to distribute the available CO2 to each plant. But, with this arrangement, I don't think that you'd want any sort of bought, baked and sterilized "aquatic soil" -- you'd want real dirt, full of natural bacteria.