Fill it with water and add the fish? I don't get what you mean... Like what combination of these fish? Clarify. Personally, I wouldn't go with any of these except maybe an arrowana. A big Fahaka would be much more to my liking.
its gonna be tight... but if your growing from small it should be alright because you'll probably want a bigger tank later anyways. but good combo is probably Arowana + Ray. Oscars can get frisky with the arowana and attack it. atleast mine did.