Most large pred cats aren't aggressive in the sense of disliking other fish, they are just really big and greedy. The main Problem of keeping them with other fish is raising them as they outgrow nearly all other fish and eventually try to eat them. And younger pred cats really like to try to everything, sometimes even if it is to big.
Once they are more or less fully grown and mellowed out a bit, you can keep them with anything really that is to big to eaten and greedy/eating fast enough to compete with them for food.
Alligator gars aren't the best choice, definetely hardy and large, but they can be real jerks that like to bite.
Peacock bass are greedy, and if they are big enough to be not considered prey you can keep them with them, just be aware that cats are active at night while the bass rest. Same for lots of other typical predatory fish.
Stingray are subobtimal, as they eat rather slow, and the cats will eather steal to much or become to fat with them.
If you want to go really large, there is always the Arapaima. And other pred cats with a similiar size go well aswell.
How big is your RTC and tank?