I was thinking of doing the containers modular, would mean that the transporter carrying 3 tanks would consist of 4 different parts (1) back/engine part, (2) 'connector tank', basically the tank that's connected to the back of the ship, (3) middle 'neutral' tank, and (4) front tank. Of course this could easily allow you to make ships with two tanks, but also larger ones with 4, 5 or 6. But with the longer ones a balance thing might become a problem, unless you put the base elsewhere. But it would allow for easier modification of cargo, meaning I could simply make different connectors. Not a bad idea in itself actually!
The only major problem with the tanks would be how to make the connector and still have it easy to cast without undercuts. I've got a feeling it would be easier to have single piece ships with different load outs. Besides, I've got a feeling most people would either make ships with 2 or 3 tanks anyway if they have the option. 4 or more and you've got balance issues (not only physical balance, but also aesthetically), and the single tank looks a bit too tiny. But I need to think a bit more on this...