I recommend putting a stage strap on your "pony" rather than strapping it off center on your single back gas cylinder. This will allow you to choose, on a dive by dive basis, whether you side mount it or sling it. Really there is little difference between the two. Side mounting a AL30 is more or less just slinging it plus a bungee to pull it in tight under your arm. I choose sidemount, since your intent is to not normally need to use it and the bungee will keep it more steamlined for less drag. Plus it won’t be flopping around when you move or roll over. On the other hand, simple stage slinging can give a little better access to the knob and the second stage you will have tucked into the side of the tank with rubber bands.
To me, mounting it to your primary tank makes it virtually inaccessible when needed, puts it out in the open causing drag, and throws your off balance to whichever side you mount it on. Also, you will have to have that 3rd second stage over your shoulder and clipped off somewhere which can add to confusion if you need to switch to it. You will either need to have the "pony" turned on throughout every dive (meaning you could accidentally breathe it down or otherwise drain it unintentionally during a dive, or you may forget to turn it on before the dive and then can’t reach it to get air when you need it) or be able to reach and quickly turn it on when you need it (which won’t be easy or maybe even possible with a little tank behind your back).