A 55 gallon should be fine, but bigger is always better.
They do get pretty long, but not that thick around- just provide a tank with enough swimming space when they go crazy
They will somewhat school with eachother- so of course they can be kept together, but they do just fine by themselves.
And Shade's right, they can be tricky with feeding. Try freeze-dried krill to begin with, silversides etc. If they don't take to them, then try instilling a starvation period- they're not that great with this compared to most preds, so having small feeders on hand helps. I've seen some go straight after market shrimp, and some not eat little for months before going after only feeders. Like bichirs, they tend to eat more than they can handle- which really, isn't much compared to a senegal. Usually, they do a "death roll" to tear off chunks
Word of advice though- NEVER buy them from a place like Walmart.
They're highly susceptible to a septicemia like disease. The body bruises, bleeds, etc. and the skin slimes peels after- followed by bloating with gases and floating at the top. There's no way to really treat it...
Buy from a reputable dealer at the least.