First, may I welcome you to Tropical Fish Keeping forum.
Second, I moved your post out of the other thread and started its own thread. The topics are related, but the issues are different, so this is better for you and us.
Third, to the lighting questions. T8 and T5 NO is much the same as far as light intensity (with the same type of tubes of course). T5 will be more expensive, but long-term use less energy. If you can get T5 NO tubes, fine; I can't which is why I stay with T8.
A dual tube in either will be adequate for what you are intending. As you saw, the photo of my tank in the other thread has this, and that was over a tank twice the size as your 55g.
Another option if you can find it is a single-tube T5 that takes HO tubes. One 48-inch HO T5 tube would provide adequate light.
I had to replace two of my dual-tube fixtures a couple years ago, and I tried the dual tube T5 HO for one week and took it back (too bright), and got two T8 dual-tube fixtures. These were from an aquarium store, so more expensive, but they are nice fixtures and fit the tanks well. All Glass was the brand I got, simply because that is what the store carries.