Lol after looking at these pictures, I can see how dangerous it can be if all these fishes above are put together. You go with what you want really, speaking of myself, an Arowana should be put in a big glass tank and the tank should only be for his/her alone. And admire the beauty of the fish, you can see its scales and colours, its true beauty in a glass tank rather than in a pond. Snakeheads, I am not sure, they can look too small or helpless with these giant Arapaimas and Alligator Gars. Giant Snakehead has been recorded to be up to 1.5 meter long, you see and think what would be the best. Sometimes just throwing all monsters into such a huge pond would look like a mess or may not look attractive or suitable. I wish you all the best.