In my class we were taught to plan 1/2 use for deco bottles. Use a 40" hose for the regs with a single 2nd stage. A lot of people advocate using a cheaper or lower performance reg for deco bottles. I never understood why. If you are doing any decent bottom times you will most likely be spending more time on your deco regs than back gas regs. All of my regs, as well as those of the guys I dive with regularily are as good as our back gas regs. In the event of a failure on a back gas reg. Any of the deco or regs can be swapped in and will breathe just as well.
I also plan to carry twice the amount of each deco gas that I anticipate needing. Thirds is a little conservative and thirds will still not protect you from the loss of the gas through reg failure, burst disc failure, neck o-ring failure, detaching and losing the bottle, etc.
However I also do contingency planning to ensure that I can safely complete deco with the loss of any one deco gas. On a single deco gas dive, this means ensuring you have enough back gas reserve to complete the longer deco required on back gas only. If a 1/3rd backgas reserve won't cover it, it becomes by default a 2 deco gas dive with contingency plans for the loss of either of the deco gasses, while again ensuring the reserve backgas is adequate to cover either deco schedule. If all I have is one deco gas, for example, 50%, and the 1/3 rd back gas reserve is inadequate for the lost gas contingency, I'll take along another bottle of 50%. In this case 2 AL40's are safer than a single AL 80 as the issue is not just volume but rather redundancy.
This type of contingency planning also means you will have the option to hand off one of your deco bottles to an out of gas buddy - which will leave you on one of your lost gas contingency deco schedules. But on the other hand, if your buddy is also planning his gas properly, he would need to have experienced multiple failures before he even needs to consider coming to you for deco gas. If your buddy is not doing proper contingency planning, you need to find a new buddy.
From that perspective a 7' hose on a deco bottle makes no sense. If on some rare occasion you need to donate gas from a slung bottle, just hand it off.
Deco reg wise, the idea of using "low performance" flow by piston regs is not an issue of performance but rather an issue of the design being very simple, very reliable and very O2 freindly in having no high pressue spaces past the HP seat with no dynamic o-rings exposed to high pressure O2. Performance is only an issue in that it is not an issue as by definition a high percentage O2 reg is going to be used 70' and shallower.
The other argument you see in favor of a flow by piston deco reg is that they are fairly flood freindly so that in the event of a reg failure, you could theoretically swap one to another tank. Diaphragm regs in contrast are less tolerant of having incompressible fluids in them when they are pressurized. Personally, I think that argument is pretty useless as swapping regs is not something that you need to do if you did the gas planning properly. It is also a lot harder to actually do than it sounds, especially if you add in current, low vsibility, etc. It is also not something you can practice as the regs have to be taken apart, dried and cleaned each time they are flooded. Plus, if you are swapping areg at depth, you run the risk of dorking up your whole team's gas plan and/or deco schedule while you screw with swapping a reg.
However there is a derivative of this argument that is valid. Most divers dive with the deco bottle turned off and if the purge button gets depressed during descent you could both purge the reg and then flood the first stage as you descend deeper and force water back into the hose and first stage. This will never happen with a back gas reg, but is a possiblity with a deco or stage bottle reg. I have also on one ocsasion had a deco bottle depressurize and have the DIN connection come loose a turn during the dive, flooding the reg from the inlet end of the reg. In that regard, a flow by piston reg also makes a lot of sense on a deco bottle as flooded or not, it will still work reliably with no risk of a ruptured diaphragm.
In recognition that I spend as much or more time on a deep dive breathing off a deco reg as I do my back gas regs, I use a high performance adjustable second stage (G250) to ensure that I get low inhalation efforts even with the unbalanced flow by piston first stage.
On a stage bottle that may be used as a 130' deco bottle, for travel gas or for bottom gas, I'll use a high performance first stage as well as a high performance second stage. For a stage bottle used for bottom gas you want performance equal to your back gas regs and it makes sense to have a stage bottle reg that can do whatever you may need to demand of it.
Consequently, I will use Mk 17 G250's for back gas and sometimes for a stage bottle, although I prefer to use a Mk 9 G250 on the stage bottle as the Mk 9 is both a high performance balanced first stage (a Mk 10 with fixed rather than swivel cap), very simple in design and much more flood freindly than a diaphragm design.
