If it was my tank I'd be happy with what you've got already. I keep mine in a 75 gallon with a Crenicichla minuano and they've recently gotten a lot more aggressive towards each other. I'm getting tired of torn fins.
I plan to keep a black shark, a blue channel cat, an oscar, a few pleco, and either my pair of GT or 1 of my pair of JD in my 125 when I get it up and running. I just haven't decided if the GT pair or the JD pair will be the better match.
Depends on how much you like your GT.
A Jaguar is going to be the boss of that tank. And my also start to bully your other fish, since it's a 120. A Jag really needs more space if you want to keep it with tankmates.
i would not keep any pairs of fish. If two of them pair up for breeding, kiss your other fish good bye. My jd pair almost killed my gt who was 3 in. larger then both of them. I had to relocate them.