It is possible to disrupt an enemy empire with spies alone, but if you've only got low valour spies, you can really only do it in naturally-rebellious provinces (eg: Estonia), or provinces with small garrisons that you've already primed with your Imams.

The best way (that I know) to use spies to foment rebellion is to use them in conjunction with Imams and an invading force... and you've gotta plan ahead for it to be really effective.

1. Plant Imams in as many Russian provinces as possible for about 10 years
2. Plant spies in as many Russian provinces as possible for about 4-5 years.
3. Take and keep two (or more) of their provinces in one year.