It's my understanding that these decisions are usually made person by person by the career managers. I'm not too sure whether it's a broad policy solution or more of a personnel management solution.
I would also say that, to your last question, when Shannon brought up the special needs, I was reminded of a point that I wanted to make about the families who get separated because of their family needs. When I was working in Brampton, there was a gentleman posted to my unit who couldn't bring his family with him because he had two children who had special needs. He ended up separating from his family and seeing them only on the weekends.
That's actually pretty common—families that are separated, with the parent living on base during the week and then maybe having a chance to go home on weekends.