Agree with the others about plecs- its just what you have to live with if you want them or any other large messy fish, feeding it less to stop it pooping isnt fair on the animal, its not like they do it to spite you :lol:
In my 460L i have an Oscar, Sev, 3 silver dollars, a common and a RT black shark and i do 50% or so weekly with a decent vac. I may not *need* to do such large w/c's but i do just because- clean water means happier fish.
I would also steer clear of UG filters with oscars in a tank (or other fish that dig)- they dig 90% of the time and that will render it nearly useless. Personally i would prefer to remove that waste than waiting for it to break down slowly.