I think anyone who's seen Lieutenant-Colonel William Fairbairn's books would figure that a trained soldier can be particularly dangerous, far more so than a convict, who is, after all, an amateur (no matter how enthusiastic) at fighting. During WW2, Fairbairn trained Canadian, British, and American troops to use things like the British-issue Brodie helmet as a deadly weapon. He was apparently a veteran of something over 600 genuine fights, and had used his knowledge of Juijitsu and Chinese unarmed combat to train the Shanghai police from the early 1900s to 1940. I seem to recall that he, in collaboration with Eric Sykes, developed interesting techniques which enabled a pistol shooter to hit targets in the dark. And of course the two of them designed the Sykes-Fairbairn fighting knife.
Unless all that sort of stuff is considered vocational training in prison, I don't think I'd bet on the jailbird.