There are 9 recognized species of Electric Catfish at this time. Finding Malapterurus microstoma (Dwarf Electric Catfish) is almost impossible, I've only seen 1 or 2 members ever that had them and I believe both were in Asia (SGP). The dwarf variety appears to have a more elongated/sloped head but other then that I don't know of any easily identifiable characteristics.
The commonly available specie is Malapterurus electricus and gets close to 3ft although I've not seen anyone with a captive one over 18" and even that size appears to be rare in captivity. I have a bunch of threads on MFK about my previous experience with them as far as food preference, aquascape, and tank mate compatibility which is VERY limited. The only confirmed fish that are immune to the shock are Synodontis specie catfish. I read that they could take it, tested the theory myself, and they survived just fine for the year or so I had my E-Cat. Other fish that were tested were killed the first night in the tank. I kept Synodontis Flavitaeniatus and Synodontis Multipunctatus.
Yes they can shock you. The shock grows in power with the size of the fish. At 12" mine had enough shock to make your arm go numb for a bit if it hit you hard. Yes they can stun/kill tank mates if they choose. I'd say putting in an Elephant nose which isn't a true electric fish but that has some electric feedback will end in a dead Elephant nose almost certainly because the E-Cat will probably fry it.
As far as food I never observed mine to eat live fish. I fed it frozen blood worms, market shrimp, live night crawlers, and beef heart on occasion.
For any more information outside of my old threads you might try the link below to Planet Catfish.
http://www.planetcatfish.com/common/family.php?family_id=16
~Trent