If you are going with a 55 gallon, I would only get one Oscar, and really they do better in 75 gallons. I don't think most people realize they can get up to 16 inches long.
Oscars get a bad rep. Most people think they are aggressive, but really they aren't. I eat smaller fish doesn't always = mean fish.
Some compatible cichlids include Texas cichlids, green terrors, severums, fire mouths, and convicts. Some non compatible species are Dovii, red devils, red terrors, or jaguar cichlids. But honestly, IMO the best mate for an Oscar is another Oscar. They pair off, so I wouldn't recommend odd numbers... but even if you keep even numbers over 2, you could end up with one pair, and two singles... and no matter what tank mates you go with, make sure they are at least 2/3rds the size of the your Oscars. (Mine tried to eat a pleco about half its size and almost died because of it...)
Like mentioned above they are messy fish, so make sure your filter can keep up with them.
I am by no means an expert, but I'd be happy to answer any other questions you have about Oscars.