I think these guys were assuming that you were talking about a single-bulb pendant. That's also why they were asking about what light is 450 watts. No single MH bulb comes in that size. It would help if you listed what configuration you have. How many bulbs, and what wattages? Also, how long (wide) is the lighting unit?
Regardless of that, if you have 400+ watts of metal halide spread across two or more bulbs over 55g, you are fine, and at 20,000K, you won't need supplemental fluorescent, but beware, 20,000K doesn't make the tank particularly good looking to the human eye. If you wanted to strike a nice mix of good for viewing and good for coral, go with 14K.
Also, if your light system has more than two bulbs, make sure a bulb isn't shining down directly on the center brace of the tank or you could melt it... which would NOT be good.