I read that anubias originate from Africa, might want to look into some of those.
I have not found any true floating plants from the area.
I too am dreaming of an elephant nose biotope (though not as specific as a particular river).
First, I would get 5 minimum. This will reduce the possibilities of a weaker one being picked on. I think a 75 would be perfect (this is the size I plan on as well).
All mormyrids are really either great or a pain. Everyone has a different experience with the fish. The most common? The aquarist watches as the fish slowly starves itself to death. I have seen videos of them being hand fed on youtube, which throws me off a lot. But prepare yourself to sit there and devote many hours of headache to getting them to eat. They may be more willing in a group. You WILL need to feed 100% frozen/live diet. No beefheart and that other garbage either. Worms worms worms. Blood worms are a favourite. But sometimes the fish won't even eat that. Some fish will only accept live (at first at least) so I'd be prepared with some lives foods right off the bat. The fish is nocturnal so that's the best time to feed. I just realized you aren't asking for this info, so my bad, I'll leave it anyways as the seriously fish profile says they are unfussy, not true.
For the tank, you'll want around 2/3 sand, and 1/3 gravel. I'm not sure why gravel but I read that a mix of the two is best.
The Niger is naturally pretty devoid of plants, but it is murky, which is why planted is good, dims the lighting. So a planted "Niger River" biotope for these guys would be somewhat unnatural. This also makes the amount of aquarium fish pretty small. A blackwater true discus biotope would technically be more natural, with lot's of driftwood for hiding as this is a small species, huge open spaces.
Personally I was thinking more along the lines of the Congo for my tank, which would give me my two tank mate possibilites; african butterfly fish and congo tetra. A quick search says that the african butterfly is present in Nigeria, so that is a species to look into.
Hope that helps!
Also, a PFK article about mormyrids (if you haven't seen it- lists some compatible species but haven't checked if they'd be part of the biotope. The last section "Mystery of the Flashing Water" is totally irrelevant but it enamored me with these fish even more, that's for sure!): Definitive guide to elephantnoses | Features | Practical Fishkeeping