At the risk of saying the obvious you and your husband are too involved with his mother. This is called enmeshment. Planning to have her move in with you for financial reasons seems like going from a bad situation to worse. Why would both of you want to live with a person who has no regard for your feelings? No amount of extra living space is worth the aggravation and would be setting yourselves up for a disaster. You and your husband need to have a different plan for your future that includes less (certainly not more) of his mother and none of his ex. If there are no children from his ex, then the two of you need to make a life separate and apart from both of them.
Your husband telling you to grow up is also interesting. His mother is flagrant about her lack of respect and makes a point of talking about the ex. Your husband is aware of the history of her trying to break the two of you up — so he and his mom saying you should grow up seems like he wants a simple solution rather than to respect how you feel.
In order to forge a new plan I’d recommend you have a few sessions with a couple counselor. This will bring the issue in front of a third party who may be able to help. The “Find help” tab at the top of this page will help you find someone in your area.