Agreed,
oscars are a odd one. They are not really aggressive, but they have their moments and the occasional psycho member of the oscar society.
My fish grew 10" in a year from a baby at 1". So, you must figure in another 6 months or so, your oscar will be 10-12".
So, finding the appropriate sized tank mate will be interesting.
If you are good about keeping check on the water quality, you could manage a second oscar, because you sound like you really want a second fish. You would have to hope that your current oscar would like and pair up with the new one. The younger the better for this to work. You would have to get one about the same size, as well as try to make sure they get equal food, so one doesn't jump ahead on the growth curve.
I would figure at least a 50% change twice a week with two oscars to keep nitrate under control, just as I need to change mine nearly that much already in the 55 and he is just under 12". It seems I can go a couple extra days since switching him to Hikari Cichlid Bio-Gold+ (less than half the price online compared to in-store), he also seems to poop less with the same feeding with this food.
I have looked up a lot about oscars, and personal experience ... read up on jds/ebjds ... but the others, I haven't looked into much. I wouldn't go with African Cichlids, because they are more aggressive, even though usually being much smaller, and, they also need a higher ph level (different lakes, different levels).
What are the dimensions of this 100 gallon. That could make a big difference too. A 4 ft long 100 gallon, which I have seen many of, I would be strongly hesitant of a second large fish due to territory, unless the two are paired, and you have some decor to break up the tank and give a sense of personal space. A 6 ft long 100 gallon is better suited for a second big fish, since they can claim stake to a side, and still have 3 ft without invading the other territory. Full grown, an oscar will be over a foot long, so you look at a 4 ft tank and realize that your little guy only needs to travel 30-36" for his nose to hit the other side, it shrinks the tank quickly.
Caring for fish can be easy and complicated at the same time. When mixing large fish, you have to give territory, watch for individual aggression, for I have seen two oscars grow up together, and one turned cruel and aggressive, while the other was timid and gentle. Tall plants, caves/hidey-holes, and good water quality help a lot. Poor water quality is an agitator. High nitrates, or levels of ammonia and nitrite can spark aggression, even between a paired up couple. Think about how people are more agitated in 100 degree whether, or if pollen is high and they have allergies, and other outside factors that can turn pleasant people into not-so-fun to be around disgruntled citizens ... congested traffic on a hot day with a car with a broken a/c is a great example
I think with a paired up set of convicts, you will honestly be best served with no second large fish, or see if your lfs will let you try to find a mate for your oscar. This way, the convicts can establish one side of the tank as theirs, especially for breeding (they will get aggressive and protective of the eggs and fry), then the oscars will take the other side to call home for sleeping, and possible mating. The problem I see that could happen, unless you have very passive members of the respective species, is if you get another large fish, unless they establish left, middle, right territories, there may be a bit of claustrophobia-induced aggression.
Strange things happen. People have pulled off combinations that shouldn't work before. I am only trying to tell you the easiest way about it. Less fish, less water changes. Less fish, less chance of aggression. But ... if you and your LFS are willing to try, watch, and put the effort, you never know what you may get away with. Point is, some oscars will eat a new oscar introduced, some will have parental instincts and allow the baby oscar to follow it around like a puppy.
