If you are doing the Master-Cleanse for weight loss, then you'll want to try to stay away from starchy foods for a while. Or at least be very careful about how much starch you take in. Legumes have starch in them too. If I were going to add a starch to my soup, I would either do legumes or potatoes but not both.
As far as which legumes, I'm sure you will enjoy the flavor of any of the ones you would choose. Any would do nicely!