212 litres is about 56 gallons, and it's recommended that you have 75 gallons for each adult oscar. They get very aggressive when they grow (and they grow very quickly!), and you'll find that two will end up fighting. Especially with other fish in there, you'll find that it will get a lot harder to keep the water steady, and keep aggression down.
Before you do anything else, I suggest you read up on the
nitrogen cycle and water parameters. They are very important in keep your tank and fish healthy. As you have fish which will grow quite large in a tank of that size, I expect your parameters will be fairly high.
It's a good idea to research things for yourself before you buy. Don't always listen to pet store employees' advice, as it's not necessarily right.
You will eventually need to rehome some of your fish, but I don't know the size of them at the moment, or your water parameters. Let us know once you've read up on it.