Thinking about it some more, I think the effects should be scaled:
* If a race has 10 planets and you have spies on 5 of them, a rumor is 50% effective, etc.
* Sabotage is not scaled if you have a spy on that specific planet. If not, the percentage chance is reduced by the ratio as above, deducting percentages from the chance of success.
* Stealing tech is always reduced by the ratio of planets you have spies on.
* It would also be nice if your inactive spies could have a chance to pick up on a 'crackdown' the race is performing to flush out spies.
* Once you know of a crackdown, you can have a command to 'lay low' to prevent being found.
* Other races are also spying, and can have their spies flushed out and such. Spies cost them money as well.
* I would recommend to get rid of the non-spy ways of doing rumors/propaganda, to emphasize the spy system and its novelty in this game.