Between now and December, you will see minimal growth in your GT, maybe a couple of inches at most if you get it at ~2". Buy the oscar later in the year and put it in at the same time as the other fish in the newly built tank (quarantine the Oscar prior). The hierarchy will change when you move all the fish from the 75g to the newly built tank, anyways.
I do like the idea of the Jurupari, and Chocolate being added to the mix with the O. You can even put the sev in there. Personally, I like the look of a male GT over the blue acara. The chocolate will grow about half as slow as the oscar and would both would see each other as direct competition due to similar body shape, so keep that in mind.