First, kudos. I am so impressed you would be willing to share with your Mom to give your kids their own space. tha tis dedicaiton and love and real sensitivity yto them and putting them first.
But, yes, have your own room. You will be a better Mom for it. I dont' know how long kids can room together like that, so I can't help there. In general, I think it is preferable for kids to hav etheir own rooms. They can learn to share in lots of other ways. But if they need to share, they will do great.
I would suggest looking at your new home and considering how to giv eyour kids their own space. I feel especially for your four-year old girl, but I think it applies to your son too. FOr instance, do they have a playroom? Can you set up a little playhouse for your daughter to be "just hers" and somethign similar for your son. Or can you giv eher her own special chair with her name on it. I don't know if I am saying it well, but I don't think kids should hav eto share everything. I think they are entitled ot their own space, their own special toys, their own alone time. And anything you can do to encourage and enforce that, I think you should.
I thik your idea of keeping your daughter up later is a great idea. Even if it is just 10 minutes alone iwht Mom it gives ehr her own time. And it gives you son a chance to go to sleep without them riling each other up. I also would keep him in the crib for a few months. Restricting him physically will enforce the new boundaries of the new house (go to bed and stay in bed). Then when you move to a bed, he will know the rules.