When I started writing this it was mostly 'is milicent really canonically fat?' but I seemed to have wandered into the larger question of 'how much am i allowed to change their descriptions for smut fic?'

Very good question. Make her recognisable. In a Hogwarts fic I wounldn't change their character/appearance too much from canon. But as most our fics tend to show the characters at the age of 16 or older, they have grown up (and out of their teenager bodies). So, why shouldn't your Millicent be a strong and pretty woman? Even if she's probably still more crouchy than others. :)