Well 55 gallons isn't realy enough for most of the larger aggressive neotropical cichlids (like oscars) or, at least, is considered the bare minnimum. It deffinately won't allow you to mix anything that gets to a good size.
Did you want to get a single fish for the tank or a combination?
Like was mentioned, I'd suggest africans to you instead. You could make a nice mbuna tank out of that (there's a pinned topic in the africans section about them) though you'll probably want to rehome all your plecs if their commons - they will all be too big for a 55 eventualy.
There's plenty of other africans to try as well of course. The tanganyicans are a lovely group with lots of variety to choose from.
If you want neotropicals but wouldn't mind something smaller or more peaceful, maybe look into dwarf cichlids (though you'd only be able to fit a couple of pairs/trios realy), keyholes or possibly (africans though they are) kribs/pulcher.
Alternatively, firemouths, severums, festivums, acaras and similar medium-sized semi-aggressive cichlids may be an option if you are interested.
Oh and, 'just for the record', there's always angelfish or discus...
Is there anything particular you had in mind? I'd personaly go for the mbuna tank or dedicate it entirely to keyholes
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