Lovely fish, but tare you planning a tank upgrade - because a 3ft tank isn't big enough for them I'm afraid
And nope, jags are not community fish, they cant go in with angels, they cant really go in with much else in most cases - some folks get away with them in with their oscars, devils and other big cichlids, but a breeding pair would pretty much kill anything in the tank with them up to the same size as them, even if they were in a decent sized tank.
125g for a single fish according to some sites, 180g for a pair. Considering they can get to 16", that sounds about right.
Obviously yours are happy or else they wouldn't be breeding, but imagine giving them a decent 6 x 2 x 2ft tank, it'd be much better for them as they'd be able to move around easier and grow to their full potential - and it'd look fab too