I know of no way to force the ship type.
Nevertheless, you can abort the mission if they choose a ship you did not intend. Each player ship type has its own torpedo loadout, so at the beginning of the mission you could check the number of homing, mine, ECM, and nuke torpedoes in storage. If the number doesn't match the ship you want them to play, put a big_message on the screen and end the mission.
"Damn the torpedoes! Four bells, Captain Drayton!"
(Likely actual words of Admiral David Farragut, USN, at the battle of Mobile Bay. Four bells was the signal for the engine room to make full steam ahead).