In my hayday of freshwater I had two 22 inch silver arawanas and a 14 inch australian arawana coexisting in the same tank. I also had a blue arawana in a different tank. For some people they can be sensitive, but the trick was the fact that they didn't kill each other. Eventually I did have to seperate them because all hell broke loose. But It was cool while it lasted. I also had a really big tiger shovelnose and red tail cat in there, among with a few other cichlids. The aussie got to be about 16inches and fat, and finally jumped out while we were gone and the mini pinscher ate him and drug him around the house.
I now only have the 150 gallon reef tank, which I am new at. I however was an old hand at fresh.