2. I'm not 100% sure but I think unless a scenario calls for more than one Sim, it won't let you add another. I think it cancels the scenario in CAS. As far as live mode, I would think it would cancel it. I've only play 2 or 3 scenarios though, so I'm not too experienced.
This is my Too Many Toddlers scenario that I had started with just the mother and 3 toddlers that I moved into their house. Then added the 5th sim whom I wanted to be the dad of the kids and it didn't cancel out the scenario. I just can't edit their relationship to show him as their dad but I'm hoping that when I do get it done that I can then add him as their dad. It only cancels it out if you create more than what you need in CAS as it won't let you hit continue until you get rid of the extra sims.
https://i.imgur.com/vvfmpYTl.png
