Oh, thank you for the link, I see you also concluded that he must have been rather short. And digging through some older entries, I also found this thread about the nickname, which makes perfect sense. I was kind of wondering if it was simply based on looks or on things FW might have said about him, but yeah, the Manteuffel connection is the most logical. Love that Fritz seems to have changed it once he got to know him better.
(Funny enough, googling told me that Diablotin was also the name of a French chocolate candy. :P)