I'd agree, you are at max capacity once that Oscar gets large. some would say already overstocked. IMO you will be ok but will need lots of water changes later on. I just wouldn't add anything else.
Yeah I would say your about max, i would get rid of the fetherfin or severum to ease on The bio load a bit and make maintenance easier but if your fine with doing big water changes then go ahead, I personally like your stock a lot
Well I’ve never kept the two together myself but I know that all cichlids are different and some can be very aggressive while others are peaceful so it depends on how lucky you are