Just been looking into portrait packs lately, but wasn't sure if it was as easy as installing it, getting it in my game, and then continuing on the save file that I've been playing on for a while.
Not quite, the game only automatically assigns images to people when they are created, so for a save file everyone in the mansion and slave guilds would need manually assigned imaged to have one. The simplest way around this is to use the Improved Random Portraits mod, as it includes a button in the customization tab for rolling a new image. Besides that it also gets you more accurate images paired with people.
The portraits themselves won't cause any issues, they're just images in a folder. You don't even necessarily need a "pack", you could put images in yourself (I've done this trying to find images that fit a particular character well).
