If you have multiple Cylon raiders in the same zone as vipers or civil ships when the activate, do they they all fire at one target?

I've been playing that you activate them one at a time, so they fire at a target if available, but if a previously activated raider downed a viper, then the next is free to fire on the civie ship or move.

What's the correct rule?

I think you have it correct, I play it the same way. Raiders would be kind of teethless if thay all had to do the same thing regardles of what the previous ones has achived.

Yes, t's played one raider at a time, but you have to finish a "zone" on the board before can start the next (this might affect the direcetion raiders move for example).