I mount my pony upside down using a ponease mount ( see
www.sea-lutions.com/ponease)
I also now dive a bungeed wing. I take the reg hose from the pony and slide it under the bungee straps. That keeps it nice and snug and from flailing about. The reg itself is secured on my right hip with a rubber retainer. The bottle is placed in the mount such that when I let my right hand just fall back, it just naturally falls to the position of the valve on the pony. And no, I have no tendency to roll to one side because of the bottle.
One thing to be careful about....at least IMHO. Make sure you mount the bottle high enough so that when you take off your rig and set it down....the main tank is what hits the ground and not the pony valve. That is one reason I like the ponease mount as the bottle is "permanently" mounted to it...and the whole system is mounted to the cam straps by a pressure plate system. And you can adjust how high or low it sits.
A second benefit to the ponease is since it does mount to the cam straps of the bcd being used....there are not tank mounts to have to carry along for rental tanks.
I also used the system when I was diving a SeaQuest Pro QD...vest style...bcd. As for routing then.....I used a plastic clip system that hooked to a dring on the bcd and the hose would be held securely into my side by the clip. Rarely did it get pulled out unexpectedly.
Not sure about the SPG question. Do you mean you want a gauge of some kind on your pony?? I had one but found it useless. I take a couple of pulls to ensure all is working well before each dive....and after a day of diving I have my pony topped off. I personally found the money spent on a pony bottle guage to be better suited elsewhere. But if you want one...go for it.
Now it is only a matter of time before all the same people that have done it before...come on here again...and tell you that you (and I) are idiots for not slinging the bottle. It is all in how you look at its purpose. For me, it is NEVER to extend my dive time. And, it is NEVER to be handed off. If someone has a problem, thats why I have an octo. I consider every dive...even those with good buddies....to be a solo dive. Self sufficiency is critical IMHO. But there are those who disagree and will vehemently say so.......and the count down starts...............
rich