[Taera]

Very simple.
Two ways: First i hear Gold construction files no longer requie the class but the actual name of the design so it should not be a problem in gold.

Any way the simpliest way is to forbid from AI to research bigger ships by altering Research file and thus make them unable to build larger ships untill you want.

On the topic of the topic (lol):
I can say that smaller ships should not be counted out. I am not an expirienced SEIV player in any way but i am absolutely sure that several of the small ship strategies used against AI could work against human. Smaller ships are cheaper and have built-in defensive bonuses giving them an edge against larger ships that are more bulky.
Smaller ships swarm the enemy and give the advantage to lesser empire unable to support large ships. Later on large ships rule yet still a wize use of small ships, especialy if talking about point-blank escorts with seeker or max-range with WMG.

By that i want to say that i consider smaller ships to be just another tactical tool in the hands of SEIV players.