I only see one amount for the yeast, which I assume is for the 18 hour rise. How much for the 6 hour rise?

For a six-hour rise, I'd use a heaping half teaspoon of yeast. This is just based on experience using yeast over the years. How long it actually will take will depend on the warmth of the room where it's rising, the freshness of the yeast, the warmth of the ingredients when you mix the dough, etc. If you try this, let us know it turns out, please. ;o)