I found an interesting glitch. What you need to do is use the shrink pill on somebody and then sleep before the effects wear off. Doing this leaves them inside of you, but once they exit nobody can enter that part of you again.
Maybe make it so if you sleep with somebody inside you, they simply return to normal mid-sleep, which should be easy to add. And I feel like a similar glitch would happen with the part-time jobs. So restrict those until nobody is inside of you. As them growing back to normal would be very awkward in public.