Multiple xenos on station from very start?

That is correct.

Now which side of Renaissance Station fell? Romulus or Remus?

Now which side of Renaissance Station fell first? Romulus or Remus?

Did they say anything about how long the station had been "under attack"?

I would have to rewatch the movie. I'm not sure which side fell first but I have a feeling that it may have been Remus that fell first given thats where the Pathogen Labs were and the xenomorph hive was constructed around it.