First of all the multiple electrical fields in one tank is a myth, second african brown knives do not run on a electrical sensory system such as black ghost knife fish. Even still I do not recommend them for your tank as they are normally difficult to keep, tempermant wise. Most eels in that family will get of size too eat smaller tetras, your probably alright with what you have but its worth mentioning. Also becareful which spiny eel you end up with only 2-3 stay small enough for your tank. As for more oddballs I would look into some loaches, maybe other smaller catfish. There are many options do you have any other interests besides spiny eels?