I was playing a legacy challenge in a custom world and I had my founder ask his girlfriend to move in. She agreed, but when I moved her in, I saw that she already had two elder cats, was at level four...
I have uploaded two sets of sims on my page, and I enjoy seeing other people play with them and have adventures with them that I didn't think of. As others have said, the sims in the world were put there for you to enjoy however you want to. :)