They're actually Central American, not South, and most CA cichlids have water that is closer to that of the rift lakes than their Amazonian cousins, so hard water won't be a problem for the jag or JD.
IME jags tend to be a bit aggro, but if they have a spot to call their own they'll defend it but won't cause too much other trouble. Festae/red terror on the other hand, an 800L tank wasn't big enough to keep my male from killing his mate. After that he was intolerant of any other tankmates, so you might have trouble keeping one with something more mild like fronts.