Minecraft wiki entry for Crop_farming, associated text: Alternating rows of different crops still speeds growth, but as noted above, speed may not be a priority at this point. Planting the crops solidly on separate levels is more convenient for harvesting what you need at a given moment, and they can grow while you do other stuff.
The water dispensers do not harvest melons or pumpkins, but may instead destroy their stems. Accordingly, the dispensers on the melon/pumpkin level(s) can be unloaded, replaced with other blocks, or (on the top level) skipped entirely.