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Oscars are said to be like dogs that live in tanks, in that they learn who provides the food and because friends with you. I'd say you're right on 55g minimum. More is always better, and keep the water CLEEEEAN.

All the oscars you see at the pet store are the same species, just different colors and morphs. there are three types of astronotus to my knowledge, ocellatus (what you see at aquarium stores) crassipinnis, which you'll probably never see in a fish tank, and orbicularis, which you'll once again probably never see in a fish tank. Very rare fish to see in the hobby.

IMO a 75 gallon is a must, but preferably look into a 5-6 foot by 2 foot tank, as these fish need a lot of room to relieve boredom, not to mention they're very wide bodied and clumsy. They have a very hard time turning around in a 55 gallon tank. They're well known to relieve boredom by breaking things and eating things they can't swallow, like suction cups and filter intakes. These fish need entertainment, and a 55 gallon tank hardly gives them room to turn around. a 75 at least has some extra turning space. Another good way to keep an oscar from getting bored is to provide a ping pong ball, a lot of oscars enjoy batting these around the top of the tank.

Keep in mind that when people say an oscar gets a foot long, that's excluding the tail. Sometimes an socar can get upwards of 14, even 16 inches long, excluding the tail. That's longer than a 55 gallon tank is wide.

A good general rule of thumb for keeping multiple oscars is to have 50 gallons per oscar fish. But typically, like most cichlids which can be kept in groups, you want to keep them alone, in a pair, or in a group of 6 or more.

Oscars are also prone to hole in the head disease, which is caused by dirty water or poor diet. Typically a mixture of the two, since people think oscars are carnivores and give them a diet of feeder goldfish, and dont change the water often enough. A good source of Vitamin C will help keep hole in the head at bay.

People refer to oscars as buying a dog, not only because of their levels of intelligence and personality, but often their lifespan. A healthy oscar can live well over 15 years. IMO due to their intelligence levels keeping an oscar in a 55 or 75 gallon tank is like keeping a dog in a crate all its life. It's just not right.

It's hard to say what you can keep with oscars, honestly. It really depends on the oscar you have. Some oscars actually prefer to be kept with fish smaller than themselves. I've had a friend who kept their oscar with odessa barbs and rainbow cichlid, buy destroyed a green terror. It just didn't see smaller fish as a threat. Other oscars get destroyed by larger fish, but rip apart fish smaller than themselves. Commonly, oscars do well with convict cichlids and silver dollars, as well as large catfish like doradidae, synodontis, and pimelodus.