Speaking of cactus hedges, (and not knowing where you are located), I am really fond of the leafy cactus Pereskia spp. as a hedge with its lovely rose-like flowers -- but that is far taller than the Asclepias you mention. I am not sure how well Pereskia does in desert conditions, it's more of a tropical cactus that can withstand tropical wet and dry seasons.

So now I'm curious, where are you?