If with "ALL" parking positions, you mean all at once for all parkings, then no, it's not possible, and wouldn't make much sense anyway changing in the way you are asking, because sometimes it's even difficult to find space for the pushback tug, let alone adding up to 3 stairs and two loaders. On top of that, since vehicles don't have any notion of other vehicles, one way to fix most of the potential collisions, is to start then at different positions.
So, the default starting positions as they are now, are the most sensible choice that works for the majority of airports.
Of course, you can change every vehicle starting position at every parking using the parking customization feature, but you can't make a global change that will act as a new default, you have to change each parking individually. When you start doing this, you'll realize that putting stairs and loaders together with the pushback tug (this basically means, starting all vehicles from the same place) is not possible, especially at detailed addon airports.
