Isolated Baseball Pitches (Clip) "Overlays"

gotta say I looked at this post yesterday and couldn't understand what you were asking.  I guess a good night's sleep helped.  I'm assuming you've got a locked down camera.  If you stack the shots on top of each other in the timeline, you can isolate each ball by drawing a circle around it in the opacity section of the effects controls and tracking it.   Not sure if the circle preset or drawing it with the path tool would be the best.

If you need more details on this workflow, post back.   The automatic tracking can be amazing or frustrating depending on various elements.  You often need to go in frame by frame and tweak...