Welcome to mfk but I'm sorry buddy but you should focus on getting a larger tank before getting more fish. for the Arowana alone your going to need at least something around 350 gals. and thats not including the ID shark those beast get over 4'. alone. You can always try Craig's list for good deals on aquariums and supplies.
My pacu reached 32 inches in my 220 gal,and he was too big for the tank... it totally great that you asked before you made the plunge... because otherwise you would be facing so many problems down the road.. now for the arow you will definitly need a larger tank .. but not near as large as you would for a pacu..
When i first started i bought 3 pacus from petco. Not knowing what they really were or would become. In 6 months i had 3 very lg pacus in a 125 gal. Luckily i found someone who had a very large home for them. Im told they are over 2' acoross and eating whole apples.