Personally, I don't know that I would white out the pots, myself. I don't know that it is ever going to get so hot down in San Diego that additional sunlight would heat up the pots enough to damage roots, given proper summer watering, but the dark color might actually help in winter months by warming roots and plant base. Hopefully you have access to both sides of that fence. As for planting cuttings, I'd double or triple up on the cuttings, per pot, if you have enough plant stock. It will lessen the chances of you losing a complete pot worth of work if a plant goes sideways. Just space them out an extra foot or two apart. This will also allow you to wedge new plants in between the existing, in the event you end up with a type you don't particularly care for, or was mis-labeled, but you don't want to just remove due to all the work you put into getting it to fruit production age. Also, new varieties keep popping up, so you'll want to make sure you leave some room for future planting. The plants will grow as big as you allow them, provided they are healthy, but that also means you can manually control a maximum size too, and prune them down if they get too thick, so there's very little chance of 'overplanting' an area, as long as you don't put TOO many in a single pot and they have to fight for resources.
Just remember, when you fill your pots, start out at about 65-75% fill. This is for 2 reasons. If you decide to use liquid fertilizer, the types you mix into water, its ALOT easier to be able to mix a gallon of water and just dump it into a pot when you don't have to worry about it spilling over because the soil level is too high. And second, if you also plan on using time release fertilizer, its nice to be able to sprinkle some into the pot and then add an inch or so of topsoil on top. Remember, dragonfruit have VERY shallow rooting systems, sometimes being just below soil surface, so 'scratching in' fertilizer can do serious damage when added.
Finally, remember to both label your pots with the cuttings planted (don't trust markings on the plants themselves) and alternate the order in which you place the cuttings. Make life easier on your pollinators by placing the species not in groups, but A,B,C (etc.) order. Even self-fertile species benefit from pollen from other plants.