I have done an extensive research on ghost knife and elephant nose.
Most fish have different eletrical signals thus will not interfere with one and other. Most fish will also switch its signals if it gets distorted by others.
The only reason why some of them will fight comes down to dominance. Just like cichlids, electrical fish do have hierarchy
also from all the readings (not from forums but research papers, heaps of them are out there just google it), males tend to be more territorial and active and they are in general much more aggressive (i guess to become the alpha male).
i have 3 ghost knife plus 2 elephant nose. no problem.
i had 4 ghost knife but one of the bigger one was aggro (probably boy) so i have isolated him to another tank.