I have been doing lots of research on what i can keep with a Green Terror, as i have one in a 126 gal tank.
One website said about your Jag (managuense) Mix your jaguars with other large, rowdy cichlids, well the GT is rowdy, so hopefully it will work out for you,
Just keep a good watch on them, and remove one, as soon as, if they is a prob, it is good your LFS has said they will take the fish back, saying that thou, i find most will take fish back if there is a problem, and ones that don't - i wouldnt really go to.
Jags do get very big thou and can even get bigger than an an oscar in the right conditions, so you will need to upgrade at some time so your fish have a good quality of life. And the more space their have the better chance of less conflict between them both.
Hope it all goes well for you.