Congo Tetras (Phenacogrammus interrupterus) are part of the classic "African oddball" fish, of which Ropefish are part, a group with ~3 males and ~6 females would look great.
They might need "growing on" in another tank for a while to safe, but another "African oddball" that will school is 10+ Synodontis nigriventris. I have 11 that are very shy, but some keepers (quite possibly with real plants which I'm not capable of keeping alive with my very non-green fingers) will see such groups wandering around in daylight hours. In fairness mine did when I had them in my 620T last summer with my Anubias that still needs securing to bogwood, but I felt a little guilty keeping the group in a squat tank.
Leopard Bushfish are another "African oddball" that will school/schoal at times, but you would not keep them in big numbers, 4 or 5 could work well in a 55g (4-footer?).
Another option is Distichodus affinis, I've just added 3 to my Congo/oddball tank after a quarantine period, these are far more social than the bigger D. sexfasciatus/lussoso/fasciolatus that should only be housed as singletons in "normal" large tanks.
Ropefish are very social and do far better in groups of 3+, I've been tempted by them on more than one occasion, but at the time I did not have a suitable life long tank to house them in. However, I do these days just about, in the form of a low volume 6-foot tank (~181x44x40cm). Your 55g will not be fair on anything more than a "teenage" Ropefish group, these are not small fish as adults, far exceeding 50cm.
