You need to take into consideration all aspects of a setup. What will work on one tank may not on another. And, the issue isn't the plants, it's algae and fish.
Fish stores do things differently that the home aquarist. Aside from that, I don't know what type of setup they have: if they fertilize, use substrate ferts, enriched substrate, what type of plants, what type of fish and how many, how long the light is on, how often water is changed and how much, what fish foods, snails, shrimp, what filter, what temperature--all these things influence the biological system and changing just one of them can have quite an impact.
And the space between light tubes and water is important; the higher off the tank the light is, the more light is lost by reflection. This is fine if you suspend the fixture a foot above the tank top. But that makes viewing the tank next to impossible, as you have light shining directly into your eyes. With the fixture on the frame, the light enters the water and you cannot see any reflection; there is some, obviously, but you don't see it. So it is better viewing. It's fine to suspend lights above a row of tanks in a fish room, but I like to sit in front of my aquaria for hours, observing nature; I can't do that comfortably with bright light shining in my eyes. It's like watching TV with a floodlight right above the screen, aimed at you. You just can't see the screen clearly.
The fish are my first priority. Forest fish do not occur in brightly-lit habitats. They are stressed by overhead lighting. I take the view that the light should be the absolute minimum to allow the selected plants to grow, and that means deciding that some plants will not be possible. This gives the fish a healthier environment. And algae is then at a distinct disadvantage.
My worst tank for algae is the 70g which has two 48-inch tubes (T8) over it; this tank is narrower than your 55g, and with T8 has 2/3 less light intensity of the dual T5. I have less algae in the 90g and 115g, both of which have the identical tubes but have 6 inches more water depth. That makes quite a difference. And the 115g is a foot longer, but with the same two 48-inch tubes. And I have floating plants in all, heavily in the 70g as well. Yet I have fish that seldom venture out from under plant cover simply due to the light.
I had a 55g 4-foot tank years ago, lit with one 48-inch tube, and I grew swords fine. That was in the 1980's before I knew anything about light, and when the only "aquarium" light was a grolux tube which is weaker light than the full spectrum T8 tubes of today, less than half the intensity. If I were setting up a 55g, I would probably try a single T5 HO. But I would buy it from a store where I knew I could exchange it. Two years ago two of my old fixtures gave out, so for the first one I bought a dual T5 with Life-Glo 2 HO tubes. I had it over the 90g, and it was too much, so I moved it over the 115g thinking the additional foot of tank would probably compensate. It didn't. I had it on for a week, then took it back and got a dual T8. I joked that the poor fish would be asking me for sunglasses if I hadn't. I cannot imagine that bright a light over a 55g. With a high-tech setup, yes, it should work, and with a controlled light period.
There is a lot of inaccurate information out there about light and aquaria. I still read highly-knowledgeable [this is questionable
] authors writing that you cannot grow any plants without at least 2-3 watts of full spectrum light per gallon. Yet all my tanks (except the 70g) have less than 1 watt, and no one can tell me I am "imagining" that the plants in the tank are alive. Some of these same authors still write that you simply cannot have live plants in a plain gravel or sand substrate. I've grown plants in gravel for more than 20 years, and every water change I chuck out excess growth. Quite miraculous really, considering my too-little light and poor substrates.