yes, no arowana or true gar will be able to reside in a 55 gallon tank for life.
However, if you ever upgrade into the huge tank range, ive heard both good and bad stories. The arowana reaching the smallest size would be the Jardini or Australian arowana, however they are incredibly mean and territorial and not many people are able to keep them in a tank with other tankmates with the jardini destroying them. A silver arowana is probably all you would be able able to get, unless you are incredibly lucky to find a black arowana whose requirements are about the same as a silvers, or if you are outside of the US and can shell out enough dough for an Asian aro.
So, assuming you are looking at a silver arowana, they get incrtedibly huge, and way, WAY too big for a 55 gallon...you'd be looking at a tank that is 180-200 gallons PLUS. Gars don't get quite as huge but depending on the species can get way up, and are less flexible. This means it would be harder for them to turn around in a cramped tank, so a lot of width would be needed.
So, unless you are able to get a tank in the 200 gallon range, i do not reccomend purchasing either species. If you have a chance at a larger tank, I have heard of people successfully being able to keep both species in one tank.
www.aquaticpredators.com is a good site for information on both of these predators. Browse around if you like.