Raechal
Fish Gatherer
I tried a JD with my tiger Oscar and the JD beat up on my oscar non stop and they were in a 120 gallon. Might work out for you though.
If you're really keen on the GT and JD, why not drop the oscar? It would be a fab set up without him. Depends on the oscar I guess, but the JD and GT are both generally more aggressive, and could bully an oscar - then again he could rule the roost!
I think with these big cichlids, much of it is trial and error
Been in that situation when I had mine too.
Glad I've only got a community tank now! Well, not glad, cos I miss my cichlids.