That isn't an answer anyone can answer with specific numbers. The 3 determining factors will be these:
Agression levels/decorations/compatibility/territorial habits of the fish you wish to mix
water quality and how stable it stays
space... for the animals to function and swim properly
Your example of a 55 gallon oscar tank, that is done, and also temporary. Oscars can top out at 15 inches, and the width of the tank is only 18 inches. An adult oscar would have a difficult time turning around in a tank of that size, and water quality would be impossible to keep stable unless doing daily 100% water changes. I have 3 oscars and 1 standard pleco, all about 10 1/2 yrs old now, and they have a 220 gallon tank, and it is full. Even with weekly water changes, it is hard to keep it stable. I had to leave it at my ex's house, but when I was living there, I was doing water changes twice/wk to keep it stable and healthy.
When I was working at the store, a customer brought us a full grown jack dempsey, about 8 inches long. He'd raised it from a 1 inche baby, the entire time keeping it in a 10 gallon tank, doing daily 100% water changes, and feeding well. The fish was beautiful, colors brighter than I've ever seen before in a dempset, but the fish died soon after we got it because when put into a 40 gallon tank, it was obvious to see the kinks in it's spine. It was horribly deformed and couldn't hold itself upright to swim, it simply laid on its side and died 2 days later. At the store, we simply didn't have the time or money to give it the attention it needed to survive, and we also felt it cruel to force it to survive in such a condition. We had all agreed to see what happened during the first week, and had discussed humanely euthenizing it as an end result if it couldn't swim. It didn't live long enough for us to have to.
Common sense, the kind of fish, the size/dimensions of the tank, and maintenance will all be factors for what you can achieve in this hobby.
2 of my oscars that I mentioned, I started them in a 47 gallon tank when they were only 1 1/2 inches, just babies. By the time they were 6 inches each, I was doing daily water changes of 50% and struggling to keep them healthy and stable in that size of a tank. I got lucky on a good deal with the 220, and later took in the 3rd oscar that someone attempted to "throw away". If I sit and watch the 3 in there, I can see that they are cramped, even in that large of a tank. If I watch the water quality, nitrate levels are difficult to keep low.
Some fish will also attack each other if they don't have enough territory of their own. This is where the aggression levels come into play, and the set up overall. 2 convict cichlids will claim a 55 gallon tank to themselves, and once full grown, will usually attack any other fish you try to put in with them. While convicts only get to be 5 - 6 inches each, and if water changes are done regularly, the tank could sustain another fish, say a fire mouth cichlid, the inhabitants will rule it out for you, water quality never being a factor.
Does this help?