just look at the guy that posted above me: 240G for 1 fish, I know it depends on what he's keeping but we're talking about 10-13" fish here.
It's a Dovii. That's what they need. And you are taking my post out of context. What was meant was that a 55 gallon is not very big at all, and that should be taken into consideration. Just because it looks big to someone just starting out does not mean that they should overstock it. And as you get more into fish keeping (especially CA/SA cichlids), you'll gain an appreciation for tank space, and what gallonage it really takes to keep some of the monsters.
I'm also the guy that said to just go for the single GT in the 55 gal, and skip the Oscar. I have raised GT's, and know what their growth rate is. They will not make it a full year before one of them will have to be removed if they are cared for and growing properly.
I've had my current GT from 1", and he's now 4.5", and I've only had him for 5 months. An Oscar will grow even faster. By the end of a full year, you should have two fish that are 8" or more, and putting on bulk. That is not OK for a single 55 gallon tank. I say you have 6 months at best.
Or better yet, since you are planning on getting a 75 gallon down the road, why not just have the 55 gallon as a tank for hte GT, and only have a GT for now. Oscars are super common, so once you pick up a 75 gallon, you will have no problem getting an Oscar to put in there.....If you still want one after reading about other types of more interesting cichlids, which I doubt.
And with the single GT in the 55 gallon, you can get some pictus cats or clown loaches, and some tiger barbs and/or giant danios. That'll keep the tank very active, and it won't look like 4 feet of tank with only 1 little juvie GT in it. BTW, this is one of my favorite tanks that I currently have (GT, 2 pictus cats, 3 giant danios, and 3 tiger barbs). It's super active, lots of different colors and things to look at as well.