The only thing that I take into account is the size of the animal. I do have racks with smaller tubs with mostly males and racks with larger tubs with mostly females but thats only because the males tend to be smaller then the females and so they go in smaller tubs in a different rack. As far as them smelling each other and going off feed, it really doesn't make a difference. They can smell each other in a rack right next to them just as easily as they can if they are in the same rack.

Mark