That sounds like a lot of nitrates in my experience. That could have a lot to do with diet. High quality diets help keep nitrates low. It is not my experience at all that they will be creating that much nitrates.
There are no rules with stuff like these, at best there are guides. The goal is a tank that is reasonable to maintain and stays clean (particulates are not in the water, no ammonia or nitrites, nitrates under 20ppm). These guides are simply ways that should help you achieve this.
And territory is an issue anytime you have more than one, even in something like a 150 or bigger. In the wild these fish's territories are simply bigger than almost any tanks people provide them with. The key is to get them small and let them grow up together, never feed live food (increases aggression, but also introduces pathogens and is nutritionally bad for them), and hope things go well. There is no guarantee that it will work, but if you follow these guides it is likely to. If you put two full grown oscars in there, or have one and then add another, yes it will in all likelihood lead to them fighting, most likely ending with one dead if you don't split them up.
