Given that there's 5 of them, I was thinking Troy of all people would be most suited to being alone. It helps keep up the cold facade he has to the others by hiding how softly he treats his pokemon in their downtime. I think having them view him that way is better for the plot/character interactions for now. This also gives him an excuse to remain "unrestrainedly unprofessional" with his pokemon when no one is looking, assuming he wouldn't get a roommate, or that his roommate would be a no-show.

Unrelatedly, the addition of Happiny once again completes the balanced spread of EV yields you would get from beating all of our pokemon once.