...//... Any thoughts on RB in mismatched buddy groups?
Yes, highly mis-matched SAC's are due to one of the two divers having a very low
SAC. This allows them to downsize their gas load, making their kit lighter. The
problem with this is that half of the rock bottom gas reserve is not yours, it
belongs to your buddy. This can severely limit your dive when using a small
tank.
With very small tanks, diving thirds can easily violate RB if you
wish to get back to an upline or follow the bottom to shore. The good news is
that while you cannot use a pony bottle to extend a dive, it does count in RB
calculations. Thus, you can dive thirds as you seem to have worked out with your
dive buddy as long as you carry his reserve somehow. I'll probably take some
flack for this, but his reserve can be split between your backgas and your pony.
So the problem for you is to grind through the mis-match calculations
and make sure that you carry enough gas for both of you. Your dive buddy
obviously has to do the same. You guys dive ponys so you may find that not much
changes while coming up to speed on this concept. And yes, RB is a DIR thing.
'Thirds' is good enough for everyone else.
A good DM will usually make
some feeble attempt to not pair up vastly differing SAC's so that thirds will work.