In nature, a mated pair of JDs will guard an area of about 4 ft x 4ft x 4ft (250+ gallons).
Although depth is really not an issue, because fish will deal with what is available,
But if you watch the male defending a spawning site (at about 1 minute) in cenote Cristalino, he chases off any cichids to a depth of over 3 ft.
They are not phased by the live bearers that surround them, but all other JDs are quickly dispatched.
And if you notice the kind of space these fish have available in nature in these videos, you might get the impression, a 55 gal is nothing but a puddle in comparison.
Make no mistake, JD's can (and do) become large/thick fish. Below is a picture of a 10" male I used to have named "Jax". He and a couple of smaller CA's easily filled out one of my 75G tanks. He would have been miserable in a 55G at that size. View attachment 1139779
The guys are not steering you wrong, you could probably keep a single adult male in a 55 for a while but eventually he's going to need a bigger tank. A 75 would be a much better option.
If I was gonna keep a single fish in a 55 its gonna be a gulper. I'm just trying to decide what I want to do in this tank. Don't really want a community tank already have one of those.