You need to space your jumps. Pika's short hops are quite long, so you'll have to space your ledge cancels at the middle of the stage. The only reason you'd have to really use ledge cancels on the main stage (not the platforms), is getting a good edge guard.
As for the platforms, practice your spacing. Remember that Pika's short hop is long-distance. If you plan on ledge canceling on a platform, do so from basically the opposite side of the stage. If you're airborne coming down to an edge, simply practice the exact placement of the ledge cancel.
P.S. You can always shorten the distance of a short hop, the distance all depends on your momentum (ex. running + short hop = farthest distance).
