The most common species in the trade is definately Malapterurus Electricus (the big version) which come mostly from the Nile river, Niger river, Volta river and Lakes Chad & Tanganyika. These are the ones you see coming in a 2-3" to the public. 36" Max size recorded.
The second most common species in the trade which is very uncommon at least in the US is Malapterurus Microstoma (dwarf version) 22" Max size recorded. I've only seen a couple members with this species on the forums and most if not all were not in the US. Most come from the Congo River.
I've kept the larger variety in the past at grew one form about 8"-11" over a 6 month period and had it housed with Synodontis catfish. I had read online that they might be immune to the shock and tested the theory. Never had a single problem with the Syno's living with the big E Cat. They were great tank mates because they would pick up the extra food scraps. I kept 3 Synodontis Multipunctatus and 2 Synodontis Flavitaeniatus around 3" a piece. E cats love rock hiding places from what I've seen and read but mine also loved its driftwood hideout. Easily the most charismatic fish I've kept, easily learns to recognize it's owner and I hand fed mine nightcrawlers on multiple occasion without being shocked. Their shock is mostly for defense from what I saw and not actually for hunting. Here's a few pics, Enjoy ~Trent
PS: Some of the pics are pretty rough so bear with it