Ah i see. Well the problem im seeing is that arowanas, gars, and rays are not for "average joes". They are pets for experienced, well-equipped fishkeepers. If i may ask, what was your goal when you began buying these fish?
An overflow can be configured in numerous ways depending on what your needs are. If your aquarium maker is competent at all, they should know how to build and install a basic overflow and a sump tank as well.
As far as handling the bioload, thats kind of subjective. No matter how much filtration you install, you will have to do heavy, routine water changes and maintenance to keep these fish healthy. When you get into keeping fish like you have, i would afraid its more of a lifestyle than a passive hobby.