Hi there
I think that at some stage in the future, you may find that 75 gallons is a tad on the small side. You'll probably find that your nitrate levels will soar at the end of the week. You also have to remember that if you have an overstocked tank, ammonia levels might start rising, it is not just nitrate that get out of control. Anyway, I'm not going to lecture you about that. Hopefully you have got a male and a female if that's what you want.. however, you have to remember that if Oscars do breed, you could end up with several hundred fry. Having said that, the survival rate of fly is not good in a lot of cases.
I really don't think you should add any more fish, especially a pleco. You are correct, there are various species that don't get too big. However, they are very messy indeed. This will just add more waste once all the fish get big. Just bear that in mind and I wish you good luck