To be honest, I tend to go with the theory that each MS user we've seen has an ability they specialize in. They still gain two of course, and can even unlock the 3rd power, but each can't use them to the same degree of efficiency. For Sasuke, his specialty is Amaterasu. For Itachi, it was Tsukyomi. And for Madara, it's Susano-o. So, and i'm not trying to keep him weak or anything stupid like that, I don't think we'll be seeing Sasuke do the same things with Susano-o that we've been seeing Madara do.
I agree with that idea as well.
Sasuke having a smaller stabilized Susanoo would be because he cant control his potential version of PS, and yes I do think that was PS on the fox since it was completely stabilized.Sasuke having a smaller perfect Susanoo would be because he can't stabilize the larger version.