(I've heard it theorized that human space in "Serenity"--no FTL, hundreds of planets, one government--and BSG, with multiple Earth-like planets making up the Twelve Colonies--were in open clusters.)
Serenity (the Movie) pretty explicitly said that it takes place in a single solar system around a large star with ~10 rocky planets, al of which were terraformed. Given the amount of time it took to get between planets, they seem not to have have FTL.
The whole plot of BSG requires FTL (they even explicitly call it that!), and the fact that they go to Earth only a few tens of millions of years ago rather excludes the colonies being in an open cluster.
Generally, the first stars to form in an open cluster are very massive (O/B), which then serve to carve out and compress the molecular cloud, triggering the formation of solar-type (F/G) stars. The Sun was probably born in such a cluster (from Supernovae particles in meteorites), but left it around a billion years after formation. So, intelligent life would have had to develop about four times faster to look out at an open cluster.