ScoobieDooo once bubbled...
Note: A PONY does NOT mean its EXTRA air, in that if you use 2500 psi of your 3000 psi air supply, you say "Oh I want to dive longer, so I'll use my PONY for stay down some extra time." A PONY is 'dedicated' air ONLY for an Emergency Situation - to surface with. It IS NOT used to EXTEND Bottom Time! Proper use of the SPG is imperative and using a PONY is not an acceptable excuse to run out of air!
What you are talking about is using a pony bottle as a bailout bottle. A pony tank complete with regulator is carried as a redundant SCUBA. It is only used in an emergency, such as a failure of the primary SCUBA, allowing the diver to safely ascend to the surface. This is only one of several options for SCUBA redundancy. Other options include H-valves and double tanks. It is generally accepted that a pony bottle is not a good option for overhead or decompression diving.
My advice to Fly-N-Dive or anyone else considering using a bailout bottle is to do as Andy suggested, do a search on pony and read the threads. A bailout bottle is only one of several options available for redundancy. Look at them all and make an informed choice. If you decide to go with a bailout bottle, carefully calculate the air capacity required for your situation, before you buy the tank. Many are of the opinion, myself included, that small bailout bottles, less than 30 cubic feet, are in fact dangerous as they provide a false sense of security without providing sufficient air to deal with a problem and then surface safely.
With that said here is my 2-cents.
Above fifty feet of water, a pony creates more issues than it solves. In an OOA situation, your buddy is the best option followed by a controlled swimming ascent.
Below fifty feet, a controlled swimming ascent is not a very good option so a pony makes more sense, but your buddy is still the best option in an OOA situation.
I have a 30 cubic foot pony based on the estimated amount of gas it would take to sort out a problem at 120 fsw, tell your buddy that your going to surface, and then surface safely with a safety stop. And most importantly, after you get the pony test it! Go to the quarry and practice switching to the pony, ascending at 30 feet a minute, and making a safety stop. I think that youll be surprised ad how quickly the gas gets used.
The best option for mounting a bailout bottle is stage slung as this gives you the most flexibility.
Select a regulator that you would feel comfortable using as your primary; youre counting on it to work dependably in a stressful situation. I use a Apex DS4-TX40. I also recommend using DIN valves on the tank and regulator rather than yoke. Doing so eliminates the possibility of confusing the tank valve knob with a yoke mounting knob in a stressful OOA situation.
Mike