For medium sized American cichlids there's basically two main options for a 55G. First there's the CA option, which you have had several suggestions for. tHese fish (COnvicts, Firemouths, Salvini etc) are pretty boisterous and usually rather aggressive. SOme folks love that type of setup. However, if you're not sure that is what you are looking for, then consider a more SA setup.
Species such as the severum, acaras (of all types), angelfish, festivum etc. are all rather calmer and less hectic than their CA cousins. SOme individuals can sometimes also be quite pugnacious but, on average, the aggression levels will be considerably less than with the CA cichlids.
Then you have to decide (with either CA or SA) whether you want a pair of cichlids or a cichlid community, or a singleton wet pet. I would advise a beginner to get a small community, with no pairs. Breeding cichlids can be VERY aggressive and can destroy their tankmates overnight if the mood takes them. Also, with a community, you can have several characters to enjoy in the tank. If you add them whilst young, and all at the same time, aggression levels between them should be relatively low also.
If I had a spare 55G, I would go for 1 x severum, 1 x acara and 1 x angelfish as the cichlids and then fill out the space with a few swordtails and/or larger tetras (remember that most cichlids will eat any fish that they can get into their mouths). The cichlids I have suggested are all very tolerant of different water pH's and hardness, and are usually pretty hardy too.
It's great to see that you are being wise enough to do a fair bit of research before getting your fish.
I would also suggest that some species, such as JD's and GT's are simply too big for a 55G. Personally, I would never consider putting any fish that had the potential to grow over 10" into a 55G, unless I knew I could upgrade the tank when the time came.
OK, that's the end of my Christmas morning thoughts. Have fun stocking your tank !