The two oscars each have another 6" (or more) of growth yet to come, so they will easily fill out the tank in time. And adding another new fish to the already established territorial lines could totally upset the now peaceful balance.
I also agree with the others, that you may be better removing the ones that occasionally upset the balance now, to the 60 (like the JDs and/or the GT, rather than tinkering around and adding more. If it ain't broke, don't try to fix it.
I know the temptation is always to want new, but the new might be the straw that ruins the whole thing.
If you "have to" have a new toy, it should not be a cichlid, it should be something non-territorial, like some tetras too large to swallow (good luck), and if you do, you should also consider anything will new add to the bio load, and require you to up the frequency of water changes, the amount of new water needed, and more frequent filter cleaning.
And maybe before (if) adding the fish, you could set up the 60 as a quarantine tank for a few months to be sure the new fish don't bring in/cause an epidemic with what you have already.