You can incorporate runway work as early as GPP but it's a good idea to keep the length of the runs shorter than the competition length and gradually lengthen it to the competitive length as the season progresses. If you're doing short approach jumps, a greater quantity can be performed (as many as 15) because the shorter length makes it less taxing on the body. The slower velocity of a short approach also makes working on technique much easier. Because of these two reasons, short approach jumps make a perfect fit for the early season. For full approaches, I'd say 8-10 would be a good number to do. As for doing plyos in addition to the runway work, I don't see any problem with it if you adjust the volumes appropriately.