I don't think you have to worry about feedback. If you are in a small venue, you'll hardly have to amplify them at all.
That being said, the main speakers in a bigger life system are in front of the band. And in the monitoring system, there's no need to amplify the OH. OH is loud. What I mostly end up in the small venues I mix in is setting the overall level to match the loudness of the drums, especially cymbals... Which mostly is still too loud for my opinion....
So feedback isn't really a problem.
What could be a problem is that the ECM8000 is an omni mic. So it will catch anything, even if pointed at the drums. Background noise, which on a stage with amplified guitars and bass can be pretty loud, is the main concern here...