As an aside, since PT boats were mentioned--we do have PT boats equals in small craft. Small craft with internal bomb bays can carry 6 anti-ship missiles and fire them all at once, for 36 tons of cargo. The Mowang has a 10 ton internal bomb bay, good for 1 Alamo, for example. As small craft, they vastly outpace fighter endurance, and with a 6 'torpedoes' they threaten ships of vastly larger size. Ships like the avenger would fit a destroyer role well, as they can defeat small craft without risking a heavier ship. Also, dropshuttles aren't compatible with dropships with docking collars--and dropshuttle bays are VERY expensive in their own right, so there is no good transport option for swarms of dropships. Small Craft fill that role however, as you can load an absurd number of small craft into 1 docking collar.
The Rocinante is a good example of what a combat small craft kitted to be a torpedo boat could look like, since it roughly matches up with a 200 ton spheroid small craft, and has a length and crew requirement in keeping with other btech small craft dimensions. A btech version would mount 8 antimissile systems and an internal bay for torpedoes--a selection of Alamo, Antiship missiles, or Air Defense Arrow missiles to fill the cargo bay would give mission diversity depending on what level of attack is needed.