12 " pl3co must be about the max for bioload in a 55. You must have done about 2-3 70% water changes a week! Just curios what your oscar died of? I cant see your nitrates ever getting below 30-40ppm with a big pl3co and an oscar.
I agree, a 12" pleco kinda fills the capacity of that tank, especially if you don't do large regular water changes. You might be able to get by with a severum but it would require lots of work on your part, I would not house more one in a 55g given your current situation and in fact, you might want to consider getting rid of the pleco before you get anymore fish.
The Oscar hit his head and suffered nerve damage. He had siezures for three days before passing on. I only do a 25% water change every two weeks and I check the water and its always 0ppm ammonia, 5ppm nitrates, 0 ppm nitrites. I have a HOB filter that came with the tank and a Fluval 304 running on the tank.