I agree that if you want a henchy to become a PC it should be fighter (probably the biggest disconnect in DCC, a few days in the dungeon and John the Pig-Herder has now the ability to cast spells!). In the case of your under-strength henchy, I would allow him to get stronger to get to the minimum Str 9. As long as that made sense, i.e. not Str 5 up to 9, then it is believable to me.