If it were me in a situation like this I would offer her a place to stay if her relationship ended and leave it at that. If she reciprocated your husbands feelings towards her then it would be time for all of you to sit down and hash out what each of you want the dynamic to look like. Likely there's going to have to be a fair amount of give and take on every side until you have an agreement that works out well for all of you.
I think it's a good thing that you and your husband have discussed this already since at the moment it's only the 2 of you involved romantically. And I understand not wanting your husband to have children with anyone else (I feel very much the same way although it has more to do with other children taking away from the financial security of my children).
Who knows, things may never materialize with C but you've had a very important conversation with your husband and you have both learned something about each other. When you're operating from a theoretical perspective it's hard to know how things are going to shake out in reality. Many of us have been where you are now when just starting out on the poly journey. There's no rule book and few examples of how to live this way around us in our day to day lives. Likely you will screw up, you're human. It sounds like you're a person with a good heart though. Keep on talking, both with each other and with any potential girlfriend who comes into the picture.
Feel free to PM me if this thread is too much for you. I don't see predator here, I see people who are new and who are trying the best they know how to figure out feelings and situations.
Everything will be ok in the end. If it's not ok it's not the end.