Actually the random portrait mod recognizes split age folders like teen+adult and applies both tags to them so they can be chosen for either. There isn't any harm in splitting them up though and honestly I wonder how tied up it gets when it has to look through so many folders. I haven't encountered any performance problems personally.
I kind of wonder how the random portrait mod runs compared to how it is documented anyway. I need to pay more attention to what girls come up in game, because it feels like I see a low variety over and over. I know the characters are persistent and you can encounter them more than once, but I need to start looking to see what folders it pulls each character from just so I can assure my own paranoid that it works as intended.
Or I just have to start executing every captive.