I think you would benefit from doing some reading here. You will see that no one can tell someone else who they should date (metamours getting along is almost mandatory to success though), primary partners don't come first, no one "comes first" and yes, your feelings on it never working with the woman she cheated with are very valid. Cheating means almost certain death to that relationship ever working legitimately. That kind of trust is very rarely re-built with the cheatee involved. She fucked herself on that one I think.
Its quite possible to raise children, own houses, vehicle, have a career and have many loves, even a husband or wife. What isn't all that possible for the long haul is making anyone "primary." Sure, you're new, baby steps into committing to others is a valid and sane idea. Believing that you will always be number one in her life is not. Eventually everyone she dates, commits and attaches to will be as important as the next. I would wager that you will find that her loves also become important to you too!
I would suggest she put a full stop on her interest in another until you have sorted some stuff out. Rules suck (they don't move anyone forward), try some boundaries. What would you put up with, what would you request in return for her exploring poly, what would be reasonable to you and what is a flat out deal breaker. Be reasonable and expect that from her. Figure out where the line is between okay and not okay and stick with it until it changes and then re-think it again.
Read threads tagged "lessons" and good luck.
you are not alone.