The only thing I have to add to the cowgirl comments is that your husband and his SO have had 9 months to plan and dream. You've only been involved in the planning for the last 5 weeks. Of course you want things to slow down! I'm betting they didn't get to plans of babies and moving in together overnight, if they didn't why should you?
Changes to living situations isn't something to enter into lightly ever. Everyone has to be on board 100% and truly want to make it work for everyone involved. If your husband really wants to make this woman a part of his life permanently then the integration changes have to go slowly. Rather than trying to separate your husband from you and your kids a better approach might be to come over for a family dinner and to get to know the family as a whole a little better. Is there any reason why she can't be a part of family time, at least for a couple of hours on the weekend?
Everything will be ok in the end. If it's not ok it's not the end.