The reason why it is stated as 6 or more is to keep aggression down. If you have 1 oscar, it will be more timid towards any other tank mates that do not look the same. If you have been reading up on cichlids, you would realize that aggression of these fish are conspecific (belonging to the same species). So for the most part any other type (depending on overall aggression/temperment of the species or individual fish itself) can be kept with other tank mates. (ie. Oscar tank mates JD, GT, Severum, Pleco, etc.)
Not in the least bit. Depends on individual personality of the oscar. I have (this is in one of my tanks) a 10" albino tiger and an 8" red tiger, the albino is aggressive towards the other oscar and pacu in the tank. My red tiger is more passive with tank mates, but will attack my hand when I'm cleaning the tank.
In another tank, I have a 5" red tiger and 3" red albino. The red tiger is the alpha fish in the tank with a GT being second and then the albino taking third in the pecking order. Other cichlids in this tank follow behind.
Yes/Maybe as individual temperament will vary between fish (Try and find out but only do so if they are both small fish not as adults/Yes
Since oscars are so common in the trade there is much information out there. You have had enough sense to read up initially on behavior of these fish. There is not much more that needs to be known, but just keep in mind that just because you read up on them, the same needs to be done with any other fish that you are intending on keeping. Unless you are into really finding out personally what can be kept together by experimenting.
Hope it helps.