I have a 55. I wanted a few large fish, mainly I like Cichlids (JD's, GT, etc). I did a TON of research (months) and pretty much with a 55 you either have 1 or 2 large, aggressive fish or several smaller fish. There's just not room (territorial wise especially) for several large, aggressive fish. 55g is large to MOST people, but that's when you are stocking with typical "community" fish. For monster fish, 55g is really pretty small.
I now have an Angelfish community, but before I had African Mbuna. They are pretty small and CANNOT eat feeders (they are herbivores) but watching them defend their territories was really pretty cool. And nobody told my male Msobo that the 6 other Mbunas he killed were like 1/3 of his size, lol....he didn't care that he was smaller.
You really have to decide if you want lots of fish or predatory fish in a 55. And I like ghostknives, and researched them, and I think full-grown they couldn't even turn around easily in that tank size- remember, turnaround room (width) in a 55 is only 12 inches.
It can play over and over in your head that you think you can do it, you can change the water daily, keep your fish happy....but in the end once they get past the juvy stage a bunch of aggressive fish in a 55g is a disaster waiting to happen, IMO.
You could set up a small tank and breed guppies as feeders- many smaller fish will eat the fry.