For motion I would prefer 15 JS 9 JR. Unless you are talking about for a small foward in which case I would prefer 16 JS 8 JR.
Team ratings are calculated by giving different weightings for different positions. So outside defence would weigh point guards heavily and shooting guards heavily. The particular weights for each rating can be found in the game manual.
I don't know the answer to the last question exactly, but I would say that if every player had OD 7 then your Perimeter Defence rating would be inept (low).