[Senmut]

The way it seemed to me was that Caprica's primary, as we see when Baltar is gloating over Adama's ruined home, was a binary sun. Now, it would make sense if further out from the "inner planets' was a third star, making the Colonial system a trinary. It would be as if there were a second sun, orbiting a common gravitational center with our sun, beyond the orbit of Pluto. (Pluto is approximately 12 light-hours from the sun, so imagine a star about the same again. Easily reachable in a day or less with Colonial technology.) Lots of room for more planets, yet close enough to be part of the same solar system. This would work for the Colonies.
The reason I doubt some of the Colonial worlds orbit gas giants is that, at least in our experience, gas giants tend to have massive magnetic fields and enormous radiation belts. Life, at least Human life, on a planet orbiting such a world would be precarious at best.