Right, after a dull day at work i came home and sunk my teeth into this thread once more. I've come up with some good cichlids which i think would add a nice dimension to your Jack, without taking too much attention away from him. Its up to you if you get a few of each, or singletons. I just think these are a good option, and are what i'd personally pick from if it were me in your shoes.
Geophagus 'steindachneri' (6" average adult size)
Geophagus 'surinamensis' (9" average adult size)
Geophagus 'red head tapajo' (sometimes called 'orange head' 6" average adult size) (one of my personal fav's)
Geophagus sp 'pindare' (5" average size) (hard to find though)
Guianacara sphenozona (5" average adult size) (another i'd be very tempted by a group of!)
I'd recommend some Gymnogeophagus species also, but these require a period of time (usually 2-3 months) at a cooler temperature. Not sure its advisable for me to suggest these with a Jack. The 'Satanoperca' genus is another one with some cracking fish in. Albeit these are often above the 10" mark, and you may lose the focus on your Jack with such fish. Worth you having a peek at some of them though. If not just to drool over them.
A personal favoutite of mine, something i WILL be having a social group of in a 6ft one day is 'Biotodoma cupido', or simply known as the Cupid Cichlid. I had a single male last year, and by far caught the heart and imagination of all the family. Definitely worth serious consideration IMHO.
I've focused on Eartheaters as they are by far my favourite, and in a large tank such as yours i think will be alot more fun to keep. But below, so as to not be to biased i'll include a more general list of other cichlids well worth concidering. I'll try and keep it to smaller species so as to keep the focus on your Jack!
Cryptoheros Cutteri (Blue eyed Cichlid) 5" average
Archocentrus centrarchus (Flyer Cichlid)6" average
Cryptoheros sajica (t-bar cichlid) 5" average
Thorichthys ellioti (ellioti cichlid) 7" average (got a little more funk than a regular firemouth)
Hypsophrys nicaraguensis (nic cichlid) 10" (big, but i can't not recommend them as they are magnificent)
Aequidens pulcher (Blue Acara) 6" average
Aequidens portalegrensis (Port Acara) 8" average (i have two of these, beautifull fish)
There are larger fish that 'can' be kept with a Jack, i've avoided saying all those, as they get bigger and are more pugnacious. Generally won't be as harmonious as those i've listed above. Hopefully others will pop up with any i've missed
Certainly gives you some options to go away and research more yourself though, right
I'd love to see a pic of your Jack, i have a special soft spot for Dempseys