I know you're looking to stock you tank with something more than arowana [based on previous threads]
IMO - you have a couple of problems with aros
- They are large fish with large bio-load
- They have a big mouth, and can eat almost any common tropical species.
Sorry to digress the thread, but I think we should turn into "What can Larry keep with his Aro"
Because of this, it's hard to find suitable tank mates. You need to try and find peaceful, discrete bottom dwellers. Common plecos come to mind, but this gives you a bigger bio-load problem, especially when the pleco grows.
I'm not sure if there are cichlid species suitable, but they also come to mind as lower level dwellers and are probably large enough they wouldn't get eaten.
Other species I can think of would be Australian natives, or even maybe freshwater crays ???