I've been searching but can't find the answer to a question I have about cranes in general, the Kessler crane in particular.

I'm planning a shot on a walkway that's just a few feet over a brook. I want to start the shot below the walkway, which will put the camera a few feet lower than the feet of the tripod itself. Is that just asking for a tipping disaster?

If your rig is properly balanced, it doesn't matter how high or low the arm goes. It's still in balance, meaning it won't tip.

going to the max height of the tripod is giving you more chances to tip over,( only if the base is not wide enough), going low makes the chance very slim, and you can safely 'scratch' the floor so to speak.