Definitely fill in that empty space. You want to plant sororities as densely as possible to prevent chasing and give each female space to get away from the others.
This is my 45 gallon sorority, which as you can see still has some filling in to do. I have had more problems with aggression out of this sorority, than I have had with my much more heavily planted 10 gallon.
Personally, I've found if a particular female is overly dominant, floating her in the sorority in something like a breeder's net for a week or so is enough to ease some of the aggression.
I just did it today with a highly aggressive female I had and after spending an hour or so intimidating one of the other females, she has settled down a lot more.
You can actually see the female in question hanging out in her 'time-out' box.