The last spot on the right side of D will be between Barron and Mailloux. No space for both of them long term. Just like there is no space for Matheson AND Hutson on the left side.
Are you talking long term?
I think Matheson, Hutson, and Xhekaj get the left spots this year. Though Xhekaj will probably be in a rotation. (a side question, how long will Xhekaj be healthy?).
Then for the right size, I think Struble, Barron, and Mailloux are in a competition to start. There may be a rotation at first. But once the injuries start, everyone will get their chance. I think they will start with Struble and Barron in the NHL, and have a rotation on the bottom pair with Xhekaj, Struble, and Barron. My only question is whether Mailloux will be a part of that rotation. I think it's better to have him play big minutes in Laval, and only be called up if there are injuries or Barron struggles and Struble struggles on the right side.