Starter cultures from places like sourdo.com are comprised of extremely robust strains of yeasts and bacteria that coexist in a symbiotic relationship optimized for bread-making. Many of these cultures were captured by bakers hundreds of years ago who prized them for their leavening abilities, flavor profiles, and resistance to contamination from ambient strains.
I see no reason why you couldn't pick and choose different strains of yeasts and bacteria to come up with your own cultures. It will be interesting to follow your experiments. Who knows, you could come up with something extraordinary. Frankendough!