As a second wife myself, I felt the hairs on the back of my neck stand up when I read your post. Unless your ex literally has bodies buried in the basement, there is no reason you should be warning his wife against him. Whoever he was when he was married to you, he may not be that guy with her, and you don't have the right to bring your issues into their marriage. If he's a jerk, she'll probably realize it eventually. But it is far more likely that the two of you should not have been married, and maybe the two of them should be; it isn't for you to decide.
It also really isn't up to you to set any rules for your ex's wife. It is up to the two of them to determine the role she will play in their household. He is your children's father, and as such, he has the right to make decisions about how his children are raised as well. You yourself stated that she is good to your children, and I applaud you for recognizing that (I, myself, have long been painted as the villain in my stepkids' lives by their mother), so maybe that's good enough.
My husband's ex once told me that she felt bad because she had been a terrible girlfriend to my husband, and then she was a terrible wife, and then they had a terrible divorce. It sounds like maybe the terrible divorce thing also applies to you (I'm making no judgment about what kind of girlfriend/wife you were). The best advice I can offer is, if you had a terrible divorce, then try to be good parents to your kids. That is the only thread currently connecting you and your husband, so make it a healthy one. Let go of your need to get back at your ex. He's in your life until your last child has reached 18, and to be honest, probably beyond that as well. (Weddings, grandchildren, etc.) I empathize with your stated need to co-parent the children. I came into my situation thinking that would be possible, but it has turned out to be futile trying to co-parent with my husband's ex-wife. And sometimes, I guess it is. So in that case, you need to just do what you can to serve your children's best interests, and stay out of your ex-husband's marriage; it isn't your business or your concern.