Pretty much everything you were told in the Vegetable Gardening forum is right on. 1 gallon containers are not very roomy and I would avoid more than a single start per jug and even that may not be sufficient room to develop an adequate root system. And you want a soil-less potting mix, not topsoil, or garden soil - much too dense and heavy for container purposes and without adequate drainage and aeration. Also, skip the rocks - they do NOT assist with drainage and can create more problem than they may potentially solve. Cover the drain holes with screening or fine mesh to retain soil. And any vine wants to grow up, not down, so you may not get the draping, cascading to the ground growth you desire but more the tangled, vining up on itself growth described in the other thread.

I doubt the jugs will break down in a single season but do question whether they offer an appropriate size for the vines (which generally produce pretty good sized root systems to support a lot of top growth). And I don't really have any issues with incorporating some worm castings into the soil mix - it is a very diverse nutrient source and not overly concentrated in any single nutrient direction. But it may also not be sufficient to supply all the nutrient needs of your beans so supplementation with a liquid is probably advised.