In case anyone is still following this

my pony tank is working great! We had a few glitches at first -- my brand spanking highly expensive new ScubaPro reg was breathing very wet (half a mouthful of water each breath) and it turned out to be a folded diaphragm in the second stage. Harrumph! But good now.
+1 for testing all new second stages by trying to take a breath with the tank turned OFF before getting in the water! It should breathe like a brick...
It's a 6L pony, we use nitrox as in the main tank, I am carrying it as a sling on the left. It's giving me just what I wanted : 60 min dives as opposed to the 45 I had been having. It has effected my buoyancy not at all -- well, just in one way -- when I jump in, I have to add more air to the BC before I jump than I did previously.
I have gotten almost used to clipping and unclipping it myself, (I always take my gear off in the water), and the switch from my back gas long hose reg to the pony reg works fine. I'm glad I have the habit of purging my regs before inserting them in my mouth -- once I forgot to open the pony tank and the purge habit made that clear before the switch! Thank you, GUE Training, thank you, Beto!
I usually breathe my back gas down to 2000psi, then switch to the pony, breathe it down to less than 1000, then switch back to the back gas. At this point I see if my wife and best buddy wants to breathe off my back gas (we have long hoses so it's easy) and we typically breathe my back gas down to 1000. Then it's time to start our ascent.
So it's working great! Just what I wanted it to do.
Handling two SPGs is a bit kludgy but I'm working on that.
Thanks,
Bill