As far as carrying it around on the surface, do whatever's easiest, especially shore diving. Sometimes that will mean donning it at the truck. Sometimes that means bringing it to the water's edge first, and then returning for it with your gear on. Donning while seated or floating is easier than donning it while standing on sand or rocks. I haven't used it much from a boat, but I would don it while seated if doing a giant stride, or in the water if doing a backroll. The issue with donning in the water is that the tank is negative when full, so don't drop it! Of course, sidemount divers do this all the time, and I get the sense that they rarely drop a tank into the ocean.
The Hydros BC has a chest strap D-ring, and a left hip strap D-ring, right? That's close to the typical clip-off locations for tech divers using deco bottles. If you're good with your hands, I'm sure you can rig this up in the standard way.
You want one big-ass boltsnap right at the neck of the bottle, on a short leash. This connects to your chest D-ring. The leash on top needs to be short to keep the valves and regs close to your body -- the closer it is, the less in the way it will be. But you still want to be able to see it, see its SPG, and use the valve, so there's kind of a sweet spot there -- something like a big boltsnap plus 1-2 inches of slack.
The bottom boltsnap should be large as well, and on a slightly longer leash. You don't need as long of a leash as is shown in this photo, because you won't be using multiple pony bottles. But some wiggle room is nice, a little more than on the top boltsnap, maybe 3 inches. The distance between the boltsnaps is determined by the distance between your BC's D-rings. When worn, the valve and regs are around the pec, the middle of the bottle ends up somewhere below your armpit, and the butt of the bottle ends up around your hip. Depending on the bottle, and on your body, it might extend a little bit past your body in the rear, that's OK.
I would recommend mocking it up with just boltsnaps and line (no rubber bits) and fiddling with it a little until you get the length right for your body and your BC. Then go back and redo it with the rubber bits. The vertical rubber handle is nice for handling the bottle in the water (above water, the valve is a better handle).
The horizontal rubber bands (or bungees) are there so that you can tuck a regulator hose into them (see below). You can stow the reg this way to carry the bottle around on the surface, and you could put it on a necklace for diving. But, instead of a necklace, I found that it was nice to dive it stowed that way. Deploying the reg from the rubber band is very easy -- the reg can be reached by either hand, just pull and it's there. Once deployed, wrap the hose behind your neck so that the reg hose comes from the right of your face. Re-stowing it while in the water a little annoying, especially if you have a miflex hose. Rubber hoses are much easier to stow in-water.
Not quite as easy to deploy as a reg on a necklace is, but I already have my octo on a necklace. That means I would take 3 second stages down, which is more than needed, 2 is plenty. If I dived solo a lot (like, more than 25% of my dives), then I would agree that removing the octo and putting the pony's reg on a necklace is a better config. But I never dived solo that often, and I didn't want the faff of removing the octo every time I dived solo, so I left it on and tucked the pony's reg into the rubber bands.
Hope this helps, cheers.
