nice additions, id probably only get one of them until you have the larger setup. Even at a small size they can produce a lot of waste and can cause a strain on your current system.
for the upgrade, are you planning on running a sump and what not?? if not, id strongly suggest you do. More water volume and much better filtration capcity than any hang on or canister will provide, and they will not aid in the production of nitrate and phosphate, which with larger fish like that, will be an issue and you could see a lot of very annoying hair algae and other species of algae.