I am not too sure about arowana with gar. I will let someone else comment on that. I seem to see pictures of it plenty, but I don't know how well it is going. As with all of my recommendations pick and choose here. They won't all work together. Thinking of things that stay on the bottom there are lots of cool catfish. Almost anything in the doradidae family would work. Large plecos are always cool. A school of pictus might be fun. Or for medium size catfish I still like the burmese sun catfish. I have always loved a good shovelnose catfish like limas. That kind of fits with the shape of your fish. I could keep a big cat it would be a giraffe catfish.
I would think a school of large clown loaches would be left alone. Silver dollars would provide some action and should be fast enough to avoid getting eaten. Really large single tail goldfish would probably be fine. Large eels like fire/zig zag/ tire track would probably be left alone. I would think large gouramis like pink kissers would be fine in there. I always liked seeing them in ponds. Clown knife fish would work in there.
Another route to go is stick with the us native theme and do some cool centrarchids and native catfish.
