Ok, no one has even asked what types of diving are you doing. If your planning on Recreational wreck diving.( use a 19 cu attach to your bc strap with a 7ft hose)
Now if you want to do deep wreck or cave - how about taking advanced training. You might find so great info there. Try GUE- they are probably the best in tech classes. Then you would use stage deco bottles with nylon straps attached to your harness. Halycon puts out a good video ( poor directoral quality and to much ego but the tech information is the best on video) And what do you know it called DIR

Thanks EricF...I have not yet seen Irvine's video, but heard of his legendary 'ego'!
Thanks!

Oh! That video.... It confused me to death. The guy with the "ego" kept going in circles. It seemed to me like a technical home video. I’m sure for those who could understand it; it had a lot of good information.

Rebecca,
I am just a rec diver right now - just like some added 'safety margins' - just in case. I know no one should ever run out of air - but hey, its not a perfect world!

There is a lot of good information on the video. It is always good to strive to dive safer. I was just agreeing with eric that the video was poorly done.

Reverting back to the original question, for a recreational diver, a 30CF pony bottle is perfectly appropriate. In an OOA situation it has plenty of air to perform an ascent with a safety stop from the deepest recreational depth.
As far as attaching it to you and a SINGLE CYLINDER, I like a bracket system that allows for attachment to the cylinder on your back with a seven foot hose as mentioned by Eric. The bracket allows for less shifting of the tank as you move and the longer hose allows for you and/or a buddy to use it in many different environments.
If you are set on the straps hanging from your D-Rings, that is also a good method and the smaller tank is less likely to leave you off balance when mounted out to one side. In this case, the hose clamps, D-rings and nylon straps with small caribeeners on both ends are the way to go.

As mentioned above, a Y valve does not account for being out of air - regardless of who should be watching whos' air supply. We train for these emergencies and take measures to prevent them because they have happened in the past.

My two cents anyway.

Using a bracket versus a sling doesn't make one more stable or off center. The fact remains that if you have a tank on one side or the other, whether on your back or sling, you will be pulled that way. I have to set my weights to have 10 pounds on the right and 4 on the left to offset the weight of the pony I carry on my back.

However, if you want to save a couple of bucks, you can make a sling bracket from the parts at the Local Hardware store pretty easily, and much cheaper than a pre-fab back bracket. I like it on my back because it's more stable (using the Ultimate Pony Bracket) and I can quickly change out a tank without fussing with snaps.

Remember to consider what type of diving you'll be using it for and that it's a pain to take it anywhere on a plane.

Cheers