[rjandron]

I have to admit, when I asked this question, I favoured the multiple systems version. I think that the "inner planets" and "outer planets" referred to by Tigh could still be construed as referring to multiple systems in a stellar cluster.

As I recall (I think from Lost Planet of the Gods), the original colonists on their flight from Kobol had turned on their technology and reduced themselves to a primitive state of existence, where it took hundreds of yahren for them to regain even the basic level of technology. Terraforming worlds would not have been possible for them, so habitable planets would have been chosen at the outset. (Terraforming would not have been possible if the flight from Kobol was rapid, and there had been little to no time to prepare distant planets for taking colonists--given the state of Kobol at the time of the Exodus as alluded to in LPotG, I would think that there would not have been the necessary time for such terraforming).

As for how the planets were chosen, and which colony became Caprica and which became Piscera, I'd imagine that the strongest group of colonists would have taken the most preferable planet with weaker groups taking the next preferable planets out of the options available. Presumably, the Capricans would have wanted to keep the planet to themselves and would have fought off any other tribes who tried to settle on Caprica. Having a series of closeby stars would have prevented the tribes from warring with each other as they tried to rise back to a level of technology capable of interstellar travel.

Finding twelve habitable worlds in a single star system, particularly when there would be little to no opportunity for terraforming, would be a very extraordinary solar system. Almost scientifically impossible. Plus I'd hate to do the orbital mathematics to figure out how stable the orbits and planets themselves would be.

Far more plausible is finding systems that had one to three habitable planets each close by.