Hoya imperialis wants to grow in a medium chunky mix: 3 parts peat-based potting soil to 1 part course perlite, or sponge rock will work well.  It also appreciates a couple of tablespoons of ground oyster shell mixed in to the mix.  The plant needs frequent re-potting and should be fertilized at every watering at 1/4 strength in the winter and 1/2 strength in the summer.  It will grow faster with high humidity, but does not need it to survive, or flower.