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.