Ditto placing the second stage so it rides on top of the tank. If it is on the right or left side of the tank it is likely to get bumped, while on top it is in pretty well protected space and can be readily accessed with either hand.
The attached image is one of my deco bottles but applies equally well to a pony.
With regard to an SPG, I used to use a regular SPG on a 6" or 7" hose, but the hoses weather check fairly soon due to the tight bends used, it adds clutter and reduces streamlining and there really is not much reason you need to know the pressure underwater on a deco bottle and even less so on a pony bottle. In either case, checking pre-dive is essential and once you are on it, knowing the pressure will not expand the gas available or make up for poor gas planning. Consequently, I use a button gauge to facilitate the pressure check on the surface and in a pinch you can still read it under water. They have also started making a larger (about the size of a quarter) button gauge that is a good compromise bewteen small size and readibility.
The second stage is held in place with a bolt snap attached around the mouthpiece with an o-ring. This allows you to unclip it if you have the time, or to just pull it loose from the o-ring in a need-it-right-now situation.
The valve knob is placed so that it points downward. This again allows it to be operated easily by either hand and makes things slightly less likely to snag on things (although that would be a moot point with a yoke valve due to the extra knob). Plus when set up this way the tank can be slung on the right or left side with no need for adjustment or configuration change.
