i have one tire-track and one zig-zig eel. very cool fish. i have heard they may be agressive to conspecifics, but i don't think this will be a problem until they're much larger. in fact, my two eels pretty much avoided each other, and each claimed a cave at the opposite ends of their tank. of course, they're not the same species, so that might prove nothing! but the caves...they are an important point, you *must* have lots of hiding spots for these guys or they'll hide behind the filter intake, the heater, etc...and be very unhappy. rocks, caves, driftwood, whatever!! one of my mine lives in a small pvc pipe.
i've kept them in a 55g since they were about 2 inches long, and they've nearly tripled in size in a year. they'll be moving to a 125g when i move in august, but despite the eels' size, i don't think they need nearly as large a tank as another fish that might be a foot long or more someday...so a 75g would probably be okay.
i keep them with large gouramis, sun cats, a small common pleco, and a variety of loaches. no problems with any of those fish. in the past i've also kept them with zebra danios, cory cats, and swordtails with no ill effects, but it is my prediction that as they get larger they will be happy to pick off small fish like the zebras, or any kind of tetras or guppies, etc.
food? mine prefer live blackworms, but i'm sure they scavenge a bit, too, because i had my zig-zag for about 6 months before i read about them loving blackworms...i try to get blackworms and add them as a treat once a week at least.
sociability? hmm...mine come out religiously every night, poking around for food. but during the day, i rarely see more than a little mouth protruding from a rock cave or the sand.
very interesting critters though, some of my favorite fish.