A low rate is a terrible idea because it doesn't do enough. Which reaches the conclusion, this is a bad idea. 40% chance is not high enough, in my opinion, to justify risking the unit. Its going to do very little to change how Rome should be played. If you're talking AI, AI Rome's biggest problem is he builds massive amounts of warriors and attacks with those, rather than wait for actually useful weapons. He is very competent when the war doesn't happen until legions
My suggestion from a few pages ago, give them Assault (+50% vs cities). Rome becomes very good at conquering, but this doesn't buff hunting or killing units. Simple, thematic, and probably fun. Combined with cover, you can take the shock path while still having the benefits of the drill path.