hello phatpharmer,
I just did some research on the web and it seems that the sm-90 will hopefully work for treating infestations of spider mites or so it says so. I didnt read where it wanted to use soap.
Are you using neem oil also as added protection? If so I would discourage it if you can. The fewer things you spray the less will be in the plant. All these spray on solutions are systemic so the whole plant will have it in it. Neem treated plants don't taste so great either.
The sm-90 has a surfactant in it so you most likely will not need soap. But if for some reason you need to use soap...make sure you use Ivory liquid or another non anti-bacterial soap. Anit-bactierial soaps wreck havoc on your soils good bacteria.
Hope you get it solved soon. Please tell us what you did and how it turned out.
Lil Elvis