I've kept another ariid catfish with brackish water cichlids, and while it works, it isn't ideal. Ariid catfish are very gentle, inoffensive animals despite their predacious nature. While most will eat very much smaller fish, they are primarily predators of crustaceans, so as long as their tankmates are ~half their size and reasonably deep bodied, they are no problem at all.
However, they can be stressed, even physically harmed, by territorial cichlids, large sleeper gobies, triggerfish, and anything that takes holding a territory seriously. Like Corydoras, ariids seem to have no notion of territorial boundaries, and swimply swim anywhere, and that brings them into conflict with aggressive fish.
Whether you mean Arius leptaspis or Arius graeffei, these are brackish water species, so choose tankmates with that in mind. Monos, scats, and archers all get along exceptionally well with ariids, as do salt-water adapted garpike, and perhaps some of the hardy marines as well. Triggers aren't a great combination (I've tried this: the triggers hate the clicking sound these catfish make, it sounds like a threat to them, and the attack the catfish, and this stresses the catfish, which click even more.)
Convicts, oscars, and pim pictus catfish would not do well in brackish water.
Cheers,
Neale