3. I believe the the .girlsx file is only loaded upon starting a new game, which means that if you edit your current .girlsx file to add more girls in, you have to start a new game for them to show up.
However, the game should check for any *new* .girlsx files and add the girls to your game when you load, if I remember correctly. So, no problem really, unless you're dead set on having only one .girlsx file.