Since its a female I think you could do a fire mouth as well. but you might have some aggression issues just having the two of them so make sure you have different territories set up and break line of sight. It is really going to depend on the temperament of the fish. If you had a male I would say definitely no other fish. You might be able to add some zebra danios as dithers. I would not attempt the acara.
If I had a female con in a 30 I would make it the centerpiece of the tank get some low light plants. And get a nice school of lemon tetras or columbian red fins, or some type of barb.