The wisdom I heard.... Sac, Bret and Pedio all seem to take over the wort for a period of time.... but different times... First Sac...then... i think it was pedio...then bret.
I think if you ferment out your beer with Sac, then toss in a few vials (not even scaling up to the ratio of 1 vial/5 gallons)... the bugs will all take their turn on the wort and munch on their part. (assuming the alcahol levels do not exceede the tollerence of each bug)
Essentially, the Sac is going to ferment to a terminal gravity... then a while later the Pedio will have grown in enough numbers to do their thing .... and eventually eat up what they can and then go to sleep... then Brett will dominate at the finish and keep working on the remaning sugars... all the while adding more and more brett character.
It seems like sac just goes off fast, but cannot ferment everything. It starts and finishes before brett and pedio even start the race. What sugars the sac leaves behind, the Pedio and Brett go after. Pedio/Brett character ratio is governed by the available O2. The more o2, the more pedio character comes through.
I would gather with this, each barrel will kind of govern what the ratio of pedio/brett character you get based on how the staves allow the slow ingress of oxygen. So in short, the barrel itself may have more say in how your final beer tastes then the ratio of bugs you pitch at the start.
Good luck!