[Rollo]

Depending on how you'd set up a mod, it would not be much of a problem to make the AI design different size ships or better versions of early hulls.
I'd be easy to make 3 ships-of-the-line, say, one destroyer class, one cruise class and one battleship class and get a mix of these ships.

You can restrict the sizes of each class in AI_designcreation.txt and use different design type (Attack Ship, Kamikaze Attack ship, and Boarding Ship) for each size class. You can then control the build ratio of the these designs in the AI_Construction_Vehicles.txt.

For more then 3 ship classes, it gets a bit more complicated, but it's also do-able.