How does throwing across ones body affect hip/shoulder separation?
I guess you’re not a sidearmer, or you would realize that a lot of us throw across the body to some extent. This is especially true when a sidearmer will use the crossfire (a move that works only with this delivery). I wouldn’t worry about it, because it helps with the follow-through and getting into a good fielding position.
As for what you call the hip/shoulder separation—don’t worry about that either. The hips are really the connection between the lower and upper halves of the body, which is what most of us want—this is the real key to a pitcher’s power, getting the whole body into the action in one continuous motion. 8)
If you’re getting into a position where you have to throw across your body, then your hip rotation can be blocked. If the hips don’t rotate fully (relatively to the shoulders), then you won’t achieve maximum hip and shoulder separation (unless you employ some counter-rotation of the shoulders).