- Currently, the only main ways to influence races negatively without fighting them is is via propaganda/sabotage/espionage. The percentages for success for these items are far too low IMO. 90% for a well-crafted effort seems high, but with a 10% chance of failing, you'll only be able to do a few of these before you're caught and eventually the race will declare war on you.
For a well crafted plan, the percentage should be much higher -- around 96-98%. It should almost be a given that it'll work, and if you have the money to spend, it should take many, many tries before you get caught. This is the only way currently to try to take down a strong race other than declaring war on it, and even this method is slow and expensive.
The low-effort plan is also far too weak IMO. 50% is just a coin toss -- it's not even worth the money. I would recommend percentages for all plans to be pushed up to something like 70%/85%/98%.