Generally starting out big is a good idea but there is a saying that too much of anything isn't. The first question I have for you is - are you new to aquaponics or have you had a smaller unit before? If you are a "newbe" then I would go a bit smaller. You have the room in your pool for a few IBC totes and I would suggest that you try those out first to see if you enjoy aquaponics.
With the totes you can combine the two with a cross-over tube and run both tanks into two growbeds located above the lower halves using only one pump. I did a 26 tote system that works great and used 13 pumps to run the system.
With the 400gallon cut down tanks combined you still have time to catch any problems that occur without getting overwhelmed with a larger system.
If you decide to go with the IBC totes, contact me and I'll send you the plans and info you need to construct it properly. I am glad to see more interest in aquaponics, just remember to have fun and don't be overwhelmed if problems do crop up