Agreed. Standard 55 is 48x13, 50 gallon (like we have) is 36x18.
I would not recommend the Hagen fixture. I used to have one. The one I had didn't have a power switch (not really a big deal I guess, just put it on a timer), and more importantly the reflector is crap. It's one reflector for both bulbs. The Current Nova Extreme Pro that we have on the 50 is a much higher quality fixture. It also has one reflector, but at least the reflector is bent over every bulb so every bulb has its own, if that makes sense.
I don't think a 2 bulb fixture will give you very many choices for corals. A 4 or 6 bulb fixture would be much better IMO
. It's more money up front, but it would suck for you to start the tank and realize you are disappointed with the selection of corals you can keep, and then have to spend a bunch more money.
One of these would work nicely:
Current USA Nova HO X4 T5 Aquarium Lighting T5 Aquarium Light T 5 Aquarium Light T 5 Aquarium Lighting