Yeah, eventually the Jag will beat that crap out of the Con. Your idea seems worth a try, move the Oscar and allow the other fish to settle in and put the oscar back. But try with a bigger Jag. A little Jag will not be able to maintain. If you wanna start with a small Jag, let him grow out more. Put a divider so the Con will not bother it.
I know what you are saying when you say the tank looks plain/boring. What you can do is add some small schooling fish or something meanwhile to add some movement in the tank, that way it wont be boring. But there is risk of some of them being picked off by the RD and Oscar.