I don't mean to sound rude or smart mouth at all, but there is no such thing as h fastest growing zoanthid or palythoa. The fastest growers are the ones that proliferate the quickest in your particular system. Lighting, current, k value, nutrient level, type of make up water, tank maturity and parameters are the soul determining factors in polyp growth, not the polyps themselves. This has been proven many many times. What grows fast in my tank doesn't necessarily mean it will grow equally as fast in your tank, and maybe not at all and vice versa. I would advise you to buy something very colorful, low growing ( zoanthids ) which won't produce as much height and see what happens. If you are pleased with the look, I'd frag off a few 5 polyp frags and place them along the base of your tank and allow them to grow together, thereby giving you the look you desire. Just my 2 cents.
Best of luck, Mucho Reef