You have many options my friend, but If it was me, I would go for a colony of Lifalili Jewels (forest jewels) 6-8 of these would work in a 55 for life, as they get only ~4 in long.
Either that or shellies. Neolamp. shell dwellers are small, and I would toss in a few brichairdi.
Or you could go with egyptian mouthbrooders, or the often overlooked perfect aquarium cichlid Herotilapia Multispinosa (SA)
or heres my little "easiest answer" to your question.
you could do convicts if you need to feed anything or constantly have a supply of baby fish for some reason.
Or you could start with a few kribs and get many kribs. Ditto cons
Or you could go with mbuna, get a colony of something, theres lots of choices.
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The bottom line is,
You can probably keep several cichlids of the same kind,
of any kind of african or CA/SA that dosen't get much bigger than 4 inches
But you can most likley have only one dominant male in a 55.
Or you can have several males of different smaller species as long as there are no females to fight over, though I strongly oppose this as it forces all your cichlids to be celibate for life.
I would highly reccomend a colony