There have been threads in the past in which playing styles were discussed, and two of the responses have been "I don't waste torpedoes on anything under 1000/2000/5000 tons" and "the deck guns aren't powerful enough".
I've just found yet another example of real-life kaleuns going against both of those statements. When Hans-Werner Kraus, captain of
U-83, attacked the very old Egyptian steamer
Said, which displaced a whopping 231 tons, he fired two torpedoes, both of which missed, and then spent 50 rounds from his deck gun sinking the little ship. Of course it doesn't say how many of those rounds missed, but still...
http://www.uboat.net/allies/merchants/1755.html