In all honesty, Live aquaria dosnt know crap..... A fire eel will live fine with a tire track eel. Just maske sure you have more than one hiding spot for each, and get them at the same size
none of those are difficult to keep at all, most eel's get along with whoever as long as they have more then one hiding place like reefraymaster suggested, bichirs are easy to keep and really neat, I dont know anything about gars..