Sorry I'm a little late to this thread. I just joined MFK today.
As an electric fish researcher for ten years now, I can absolutely confirm that the "electric jamming" idea is indeed a myth.
This is why: First, there are two broad categories of electric fish. One category emits their electric signals back-to-back so that the resulting electric waveform looks like a continuous wave that has a constant frequency. These are known as "wave fish". The other group emits electric discharges with a longer silent period in between. These are known as "pulse fish." For the pulse fish, electrical jamming is never an issue because they simply adjust the intervals between the pulses to avoid jamming.
With wave fish, each species uses a particular range of frequencies. Within each species, individuals each have a different frequency most of the time. On the rare occasion where two individuals with the same frequency encounter each other, jamming does occur, but only briefly. One (or both) of the fish quickly adjust their frequency so that jamming is no longer a problem. (This is known as the "jamming avoidance response" and is one of the most widely studied and best understood behaviors in electric fish.) The exception is the genus Sternopygus, where they have specialized brain circuits that allow them to coexist with other fish that have the same frequency.
So, the bottom line: Electrical jamming is not a problem. Period.
The only reasons to avoid having different species of electric fish together is that some of them can be aggressive, and to avoid transmitting parasites that are harmless to one species but deadly to another. In most cases, though, different types of fish can coexist quite happily. We do it in our lab all the time, having sometimes kept four different species in the same tank.
Hope this information is useful.
to MFK! Glad to have you around!