which large(ish) SA center piece fish

i'm currently building a plywood tank that will be L shaped 4' long on one angle and 6' on the other the inside space is roughly 18" high by 18" wide. I will have some plants, mostly giant hair grass planted in little Tupperware containers and that will be secured with river rocks piled around/on top of it. I hope to have a large piece of branchy driftwood. the substrate will be pool filter sand. the fish i'm building the tank around are a group, prob 8 or so, of red head geophagus Tapajos and 2 recently purchased l091 plecs. I may choose to add a group mesonauta but that will depend on space which I don't know I will have if I have a bigger fish in there as well. so my question. I was hoping an Oscar but I am worried it might be a bit tight. what other suggestions might people have or do you think an Oscar might be okay? In a tank like this would I have room to add a bigger fish?

I've considered chocolates but, like an Oscar, was worried it would be too big when full grown. I actually hadn't considered a severum really until now. I didn't like the look but after thinking on it the idea is growing on me.
What part of the tank do they tend to occupy? And also would a sev be alright with corys? I would like to thrown a group in.

i checked out ivanacara, not really what i'm looking for in regards to this tank although a really cool fish. in everyones experience would a green terror get along with geo Tapajos well enough? I've heard they can be chill but it might depend on the individual, what do you think?

that fish is amazing....