If you are talking about Jack Dempsies in a 30 gallon you won't really be able to put any thing else besides maybe a placo or a few small corys to keep it clean
the Jacks will become to large to put much else with them in that size tank.
You can try some smaller fish but the Jacks will eat them when they get bigger.
And any more large fish in that size aquarium would be over crowding them. I can't thank of any fish that is aggressive enough and won't get big. Although their may just be something I'm not thinking of. Good luck either way.