SNIP!
If they're both in the same system and the Breen are literally wiping out one half...doesn't that make you think the other half maybe might be in cohorts with the Breen? There's no reason why the Breen will be on one planet and not the other. It's like the Trisolarians hitting a colonized, terraformed Mars but leaving Earth alone (or the other way around).
I think it would had been a bit better if, admittedly, cliche if that civilization had either gone full fascist and warmongering and attacked the cerritos with stuff like primitive antimatter munitions or stuff they brought from the ferengi or slavers, like they've turned into a sort of Kongo or Dahomey Kingdom, an elite literally selling their lessers for material gain and support.
Like they're just stewing, waiting, the Cerritos comes down to an abandoned, war torn city with evidence of war and slavery and then some dictator pops up on a screen somewhere. Mwahhaha, the Federation! Time for our revenge!