When I was younger I had an Oscar in a 55 gallon and it’s true you will need something a bit larger in the future. For tank mates I’ve had firemouths, severums, plecos, and other oscars and they have been good tank mates. It just depends on what size tank you will be upgrading to once your Oscar gets larger. Idk why I just posted on this forum I just noticed that it’s from years ago.