Less aggressive doesn't mean entirely placid. They are still territorial and can become aggressive to other tankmates.
Even as a relatively small fish, they can get to 10" with very capable teeth.
Small tetras work, but eventually can be picked off.
Cichlids, think geophagus as they aren't too dominant, but can hold their own still.
Larger bichir might work, such as congicus.
However, there isn't a blanket list of what will and won't work. Each fish is different. You might ouck out and get a very placid animal, just try to provide optimal coverage and hiding spots.