Right now you are permitted to use Centerline Torpedo mounts on ships up to 2500 tons, and later 3500 tons with the 'More powerful compressed air ejection' technology. However, there are several examples in reality of ships outside this.
RTW1 allowed us to use them on up to 3500 tons from the start.
Therefore, I would suggest that the starting limit be put back to 3500 tons, though a case could also be made for 4000 tons (Tenryu).
With the upgraded technology, the limit should probably be either 7000 or 7500 tons, as this would allow ships like Agano to be built.