I had a Ctenopoma for a while and can vouch for how awesome they are, and watching them hunt is something else. Very personable fish. I don't know much as far as Africans go, I'm a new world cichlid guy myself, but from what I've heard you can't mix much with Africans besides other Africans. New worlds will give you more personality and peace, two things I find very desirable as watching fish beat the hell out of each other is never very fun. With SA you could keep the BN Pleco, but the other fish likely will have to be rehomed due to space restraints. Knifefish definitely get too big for a 55 as well unfortunately. So given your tank space I would either do a full African tank or a Leopard ctenopoma with the BN and some smaller cichlids like firemouths or convicts, or even a school of something like Odessa Barbs. I housed them with my ctenopoma and loved their color and schooling behaviour.
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As well as what you said, Africans should, mostly, only be mixed with certain fish from the same lake, as there is 3 huge lakes in Africa that these fish come from and each is very different.
You may be able to have a Brown African Knifefish, but it is peaceful and wont work well with Cichlids as well as being a bit large for the tank, getting 8-12 inches.
Ctenopoma are preeeettty cool, i can agree. You could do these.
Some bottom feeders for an African tank could be Synodontis Cats, they aren't huge and do a great job. They are tough too. Ive heard Hoplo Cats work too, although I've not had personal experience.
Personally, i would do Peacocks for the colors, as they are really pretty, as well as being less aggressive. Im not a bug African Cichlid guy, someone like TMRC or Freakgecko can advise you better on this front.
Some South or Central American options: Firemouths, Jack Dempsey, Convicts, Texas Cichlid, Salvini. These fish are colorful as well, but are more aggressive and get larger. You could only have a few in the tank, possibly only one. However, they are also very very personable and can distinguish their owner from other people, can be taught to do things (rare, but possible) and often like to be petted. They limit the additions of other fish but make good pets by themselves, not needing a group of others to fill the tank and give it character.
As far as dithers, for either fish you are looking for similar things... Fast, active, and cheap. They will likely suffer a loss or two every once in awhile, so you want them to be replaceable. Rainbowfish, larger Tetras, Silver Dollars, and some of the Barbs are good choices. Dithers can be used to add color, movement, and a feeling of security to the other fish. They arent needed though.
Also, you say that this is your first time keeping Cichlids. Know that they are messy. The South/Central Americans (also called SA or CA Cichlids) are large and predatory, producing fewer, but larger, "craps" per day. Africans still are messy, but in a bit smaller of a way per each fish. However, when you keep a bunch of them in a tank like most people, it adds up. You will HAVE to do more than 30% per month. I would say a 50% WC (waterchange) per week, or do two 30% over a week. It is imperative to keep them in a clean environment, as it keeps them healthy, in turn bettering the colors, which I'm sure you would be appreciative of.
Also, be aware of food, especially in Africans. They need a more specific diet than other fish, and are susceptible to things like bloat more often than other species. SA/CA Cichlids are usually better suited to live off of just a pellet diet.
Also, consider what you want in the tank. If you're looking for a busy, very colorful tank, with not a ton of aggression, Peacocks are probably hood for you. If you want a bit of an interactive, slightly more intelligent pet, i would go with a CA fish, like a Texas Cichlid. Personally, i would choose the Green Texas, H. Carpintis, because it is a bit smaller than the regular Tex and is, IMO, better looking, and equally personable. My largest male is starting to recognize me well and learn that i mean food. He isn't stressed to have me in or around the tank, unless I'm trying to catch fish. He is really a wonderful fish, IMO.
So, it comes down to choosing a group of fish, selecting your favorite species or two, and building around that.
I will give an example of what i would do, as I've been in such positions before. It will be a CA setup, as I'm not qualified to be giving anything but general advice on the African frontier.
1x Green Texas Cichlid
1x Firemouth
5x Silver Dollar
1x Synodontis Eupterus
In this, i would buy the Silver Dollars first, then the Syno, then the Firemouth, then the Tex. The process of adding less aggressive fish first, in any situation, reducing the stress on them as well as gives them opportunities to get settled and not be pestered.
Also, when adding new fish, as you wont be able to add everything at once, you need to rearrange territory. When adding the Syno and Silver Dollars to the setup above, it isn't needed because they are not territorial. The Firemouth is the first territorial fish. When adding it, rearranging isn't needed. However, adding anything after it, and i will need to feed the FM and the proceed to move the decor around in an attempt to reduce aggression, as the fish wont be hungry and will be trying to claim a new territory.
Whew, long post. Thats a lot of the basics on Cichlids, as well as fish in general.. Have fun XD !!
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