Many will disagree with me but I too wanted a tank with more then my Oscar.
I ended up putting blue dolphins (same area as Frontosa) in with my 11" Oscar in a 125gal tank. If you give them room and re-arrange the tank, they will do fine. The Oscar does like to be the boss but as long as they run just like my clowns they will do fine. When I first put the clowns in they were chased by my Oscar a lot but now my Oscar for the most case likes to pick on the decor only. As for the water conditions the Oscar can deal with just about any water type. My current: pH 7.6-8.0, Alkalinity 180-240ppm, TH 50-120ppm, nitrite 0, nitrate 20, ammonia 0, salinity 2.0ms (for ick 5.0ms and 86F for two weeks). I do not add salt but for health issues like ick but the question "should you add salt to freshwater tank" I still am not sure? Oh, I did have the clowns in this 5.0ms / .3% solution of salt for two weeks and they did fine, there really is no such thing as a scale-less fish although yes clowns are more sensitive due to small scales.