I'm by no means as experienced as most on this board and try to expand my knowledge every time that I think about diving, but I did a lot of research & calculations and here is what I chose. If you are going to do it and want a redundant air source, I would go with a 30 at least. From the rec limit with some time to catch your breath and figure out what is going on in an emergency, a 30cf should give you enough gas to safely take your time, ascend, do your safety stop, and surface. Whether you decide to sling or mount your pony, I think that the most important thing is to practice in a controlled environment and know how to use your setup. Feel free to correct me if I am wrong.