As for the rest of the comments...I understand and see your points; I will analyze and compare them extensively.
I have a clear maximum limit in mind for fish in my tank, but I don't know everyone else's maximum limit. I learned various limits from various people, but I think this particular version makes sense; here it is: If the fish can turn around comfortably in the tank without the head or tail touching the sides, then the tank size is okay. I measured the width of my tank, which is about a foot long; I can push 11 if I want to, but my fish size limit is 10 inches, leaving plenty of space for a fish to turn around comfortably.
Could everyone else tell me their maximum fish size limit for a 55-gallon? That would help me more than debating what type of fish is appropriate because stating specific species is harder. There will be a different answer from everyone. At least half of y'all would say that a GT would be fine in a 55-gallon, but another half would say no less than 75. If I could get an average for the maximum size from everyone, then maybe I can work from that and find a fish that would be okay long-term in my tank.
6-7inch should be comfy.