With the exception of the Thai hybrids, plant Hibiscus on 36" centers. And if you're going to use them as part of a hedge, plan on hand trimming them otherwise you'll never get any flowers. Maximum sun, good, fast-draining organic soil (think Hawaii) and you'll get best results.

If you mix Plumbago with the Hibiscus, where you let the Plumbago grow into the legs of the Hibiscus as they grow older, you will get you blue flowers and Hibiscus flowers at the same time. And if you're really clever, you'll add a Pinwheel Jasmine to the group. If you use standard "Hedge Red" Hibiscus, you'll get red, white, and blue most of the year. Very patriotic.