My local club is going to fill a 55 wine barrel with a Belgian Brown Ale after we have each done our own primary fermentations. What I am wondering is: if we make starters of each of the individual bacteria and brett., what is a good ratio to add to the barrel. I have heard that making a starter (or using 2nd generations) of a commercial blend might result in a change in the original ratio. (Or do we even need to build up a starter?)
Can’t answer as to the ratio. Half of making sour beers seems to be closing your eyes, throwing it at the wall and hoping it sticks. 
On the commercial blend issue, my one problem with a starter is that if your blend contains things like lacto or pedio, they have different needs to guarantee optimal growth that yeast and brett. Ultimately with pedio, it’s not going to matter. stuff’s invasive if it gets established (sometimes tricky). Lacto on the other hand is a bit more finicky.
This might be a good question to propose to Wyeast or White Labs. I’m unsure of the bacterias requirements like FAN, O2 or nutrients but they may be able to lead you in the right direction.
You might also try asking East Coast Yeast. They sell a bunch of sour blends.