I am in your exact same situation, and there are a ton of post here on the topic if you search for pony bottle. I just started carrying mine a few dives ago.
Here are a few recent ones
H2Odyssey Redunant Air System (RAS) - anyone using?
Pony or buddy when low/no air?
Practice using a pony bottle
In general, if I were to summarize all the posts, what I concluded was that
1) A pony only helps for low/out of air situations, but posters listed many many other instances where a buddy would be needed. But for my dives (warm Caribbean low current high visibility guided group boat dives) air during a 100' wall dive is my primary (but not only) concern.
2) you need to calculate your air requirements based on how deep and how "difficult" or physically taxing your dives. (You will need to know your SAC rate, then compute air used for 1 minute at depth to get sorted out, then air to take you up to safety stop at 30-60' per min, then air at safety plus surfacing, then double the total to account for breathing under stress. For me a 13 cf pony had air to spare, but for about the same size and weight it's worth going to 19 cf). A spare air could be useless for me in my case. If I were not sure about my SAC I would go with (maybe rent?) a 19cf and then run a simulation from depth with enough air in your main tank to see how far the pony takes you and then double consumption for stress.
3) all of my dives are essentially non-buddy group dives, so the pony does provide some safety net for bad or non-existent buddies - and that's why I dive with mine. Don't include the air as part of your gas plan, though.
4) slinging it along my left side is the best for me, rather than mounted to my main tank and that seemed to be the moderate consensus
5) regardless of the tank size you will hardly notice it once in the water. Some of the tech divers might suggest a 40 or even an 80 but that is probably overkill for normal group recreational diving. Yes you can die if you needed that extra 10-20 cf, but you can die with doubles as well. It's diving.
6) practice using it! It seems very simple to change regs (and it really is - I keep my pony reg attached to a D ring under my left shoulder where it is very easy to reach) but it still helps to get comfortable doing it. I practice using if during my safety stops. (But alert the DM that you will be doing this!)
7) you will need to recalibrate your weighting and maybe trim. I found no effect on trim, but my first dive with the pony was a rocket ship to the bottom, and I like to do negative entries to keep my total weight down. So I'm going lighter when using the pony (I don't use it on shallow dives that I can CESA if needed).
8) Oh, and there was no consensus as far as I can tell on whether to leave the pony tank valve closed (but pressurized) or open. I leave mine open as that is one less thing I will need to deal with, but I have my seconds stage tuned down to prevent free flows, so need to turn that lever up - although last time I did not just to try it, and it breathed OK.
