Your new aro is going to grow about an inch per month for the first few months. IMO your going to find that 20g is too small for the arowana around the 5-6" mark. I wouldn't suggest moving the arowana into the 300g until 10-11". You'll have to decide what to do as there is a 5-6" void within the recommendations. Young arowana don't do well with lots of surface current, so the three HOB filters may become an issue. I would also recommend before it comes to completely seal and secure the entire top of your tank. This means every little hole and even the HOB filters, I had an 8-9" silver end up inside an AC110. You'd actually be better of to just get a sponge filter for the growout tank. Most silvers that end up dead, are from drying up on the floor outside their tank, don't let it happen to yours.
For feeding I would start with freeze dried blood worms and then add broken hikari gold cichlid pellets. Always had better luck going with breaking up the large sized pellets instead of feeding the small pellets, just a little softer. You may also want to add some floating plants to help make the aro feel a little more secure.