Would a 75gal tank be enough to hold a male and female though?
Some would argue against it, but I say go for it. However since this is your first time having an aquarium, I would just get one oscar to start. Although you can get a pair and if they breed just don't be disappointed if the fry don't survive.
As for keeping a GT and Oscar together, that's totally up to you. I have a 75 with 1 Pleco, 1 GT, 1 JD, and 1 Tiger Oscar. I'm going to give my JD to my teacher though and eventually if I must I'll have to sell my GT and Pleco, but as youngins, you can keep the GT and Oscar together, but be warned that it is not likely you can keep them together forever. Tiger Oscars are beautiful fish, so me, being a total Oscar fanatic, would recomend an Oscar over a Green Terror (GT). Especially since they are really hardy fish and so much fun to keep. And if you take the path of keeping an Oscar and Green Terror together as youngins, make sure that the Oscar is bigger than the GT and allow the Oscar a few days in your tank before adding the GT, because as young fish, GTs are more violent than Oscars.
ALSO! MAKE SURE TO DO WEEKLY 50% WATER CHANGES AND HAVE A GOOD CANISTER FILTER ON YOUR AQUARIUM! I would recoment the Rena xP3 canister filter and an HOB filter as well just so that you can place pure carbon or nitro-carb into the HOB to create more filtration to keep the water cleaner and less stinky. :yuck:
With everything said, if you can get your hands on a 100 gal aquarium instead of the 75, I would definitely say you will have a better chance keeping 2 oscars in the bigger aquarium! And you could, possibly, keep a GT and an Oscar together in a 100 gal aquarium as well. (Though I think other people would disagree with me I say it's fine and since you are new with the hobby go ahead and experiment)