I tried starting out with a sim I'd already played with and had stored to my library. He had quite a few skills and a reward trait, none of which showed up in CAS, but which did appear when I started playing. I don't think there's a way to tell if a sim has skills and traits until you start playing. The twin option does give you a clean slate, but the clothing and hair are different, and you have to assign traits yourself. Overall, I think it's probably not a good idea to pull sims from the Gallery, as people sometimes do post sims that they have played with in a game.