I don't think a 55g is big enough for any of those fish really...
Elephant nose grows to 9-14 inches
Butterfly Fish 4-5 inches
Leopard Fish 6-8 inches
African Knifefish - 8-12 inches
Senegal Birchir 10-14 inches
The 12 inch width on a standard 55 really limits the size fish you can keep. IMO, you need a bigger tank.
Not sure where you're getting your numbers from but some seem to be over exaggerated.
Gnathonemus petersii (elephant nose fish) - 9-10 inches
Pantodon buchholzi - 4-5 inches
Ctenopoma acutirostre - 6 inches max for most specimens
Xenomystus nigri - 8 inches max for most specimens
Polypterus senegalus - Depends on the genetics but most captive bred specimens (the ones available at most stores) max out at around 8-10 inches with some rarely getting to 12 inches.
I will be turning my 55g into an African Biotope oddball tank. The entire tank will be designed with an elephant nose fish in mind. I bought one previously, but it just could not compete with my botia loaches in my community tank, and starved to death, even though I fed it live blackworms every night. I have never been so sad to lose a fish I had for so short a time, and I want to give the species another try in better circumstances. So because of this the entire tank needs to be rather sedate, deliberate feeders. I will be getting my elephant nose among the first fish in the tank and I won't really add many other fish until he is feeding well. (I'm not worried about the cycle because I have an mature filter ready to go in, and I can put some fish from other tanks in initially to make sure it is ok.)
So here is my stocking list. I have never done a true biotope before other than a native tank, so I'd appreciate some feedback. It will be somewhat planted, but mostly dominated with driftwood.
1 Elephant nose fish - (another possibility is getting three or more, since they do well in larger groups, and having less other fish, but since they are kind of expensive I'll probably stick to 1)
1-2 African Butterfly fish
1 Leopard Bush fish
4-5 True Upside down catfish
1-2 African filter shrimp
Those are the fish I'm pretty set on, but here are some I am also considering, but would like some feedback on.
African Dwarf frogs (would these guys get eaten? If there is a reasonable chance they won't I'd like to get some)
1 African Knifefish - (heard mixed things on whether these are compatible with EN because of being electrical fish - most seems to say they would be fine, it the BGK that would be a problem)
3 Kulhi loaches - (These are from a tank I am taking down, so they could go in here or the other 55g community tank with lots of Botia, rainbowfish, tetras, loaches, etc.) Even though they are not African I am considering putting them in here because they are so laid back. What do you think?)
A school of congo tetras - I like these guys, but I'm afraid they will be too aggressive of feeders? Anyone have experience with them in this type of tank?
A Senegal Bichir - obviously I realize that if I get this guy the ADFs, Kuhlis, and maybe congos, shrimp, and UDC would be out.) Anyone have thoughts?
A senegal bichir would do great in a 55 gallon but as you said, the smaller species would definitely be out of the question.
Ctenopomas, elephant noses, and african butterfly fish can be territorial within their species so unless you have a larger tank, I'd only get one of each.
The upside down catfish are aggressive eaters and might not let the elephant nose get it's fair share. Same with the congo tetras.
You bring up a good point about the african knife and the elephant nose. You could probably get a better answer from MFK about their compatibility but my gut says they probably wouldn't do well together.
I'd go with one elephant nose, the senegal bichir, and one leopard ctenopoma and that's it.