Its funny you should say that about the arow, I also have a fire eel, balas, hujeta a giant gourami and a sailfin pleco. I have on many occasion seen hujeta for instance at 8inches livig quite happily with a 24. Inch arowana. I don't want to contradict what you said but there is a level of contraversy as to if the arow us only fed non live food, grows up with the other tank members and there not the size of tiny feeder fish it can be compatable.
As to the gar question I was thinking either forida or crocidile but I'm going to wait for the others to reach full size to assess space.
Finally, my current tank which I suppose you could call a grow out is 6x2x2 and is fine at the mo as all fish bar the gourami are currently quite small. I'm well aware of the long term housing needs of the fish and as such I'm moving to a house with a concrete floor and getting n 8x3x3 built for 3.5k which was the best price I coud find. If anyone feels like donating a larger tank or the money plz feel free saves me working my ass off to get it