Author of 300+ articles in Fortune, Forbes, Time, Mashable, TechCrunch, Entrepreneur, more. I've screened thousands of pitches, plus I run a firm that does PR. I know what works and what doesn't.
Any consultant capable of executing your idea won't be interested in stealing it. They're too busy with their own zone of genius to bother trying to jump on someone else's. Any consultant interested in stealing your idea probably isn't capable of executing it.
Worry more about speed and execution than whether someone is going to take your idea, and you'll end up ahead.