"Altitude Selection" is an interesting choice of words. Suppose we engineer plants to live at various altitudes and just let natural selection take over. The plant that finds the best conditions to support itself will tend to populate the planet and overwhelm all the others. No engineering required beyond the initial 'seed' population, with traits as diverse as possible.

Plants might need pollination to reproduce. How much pollen would be needed to disperse around the entire planet and find other Venusian Air Plants? They would have to have very large amounts of pollen, themselves buoyant, or a polination vector of some sort. Maybe insects, but that would be an additionally complex level of symbiosis. Such symbiotic relationships would require mutual selection to come into being, and it might not be possible to engineer them.

Perhaps collisions could be a pollination vector? When one air plant crashes into another, it disperses pollen to its victim, assuming there isn't too much damage. That would minimize the amount of pollen needed. Fruit would have to drop off in a developed state, fully capable of surviving. Maybe the fruit could take a few branches with it, with enough methane floatation sacs already there. So reproduction might consist of collisions, followed by a debris cloud, where each fragment grows independently.