Are you actively TTC or are you going to take a month or two off while you try to progesterone? I have heard that sometimes going on progosterone for a couple of months can be really helpful in shortening your cycles. I guess it retrains your body or something.
It's up to you what you do. A compromise might be to chart, temp, check your cm and cp and maybe use opks to help you know exactly when you ovulate and THEN take the progesterone cream after that ovulation has occured. That way you have a chance to get and stay pregnant. You'd have to continue the cream through the first trimester though. (I think.)
I guess I'd have a long talk with your doctor to find out exactly what it is he's trying to accomplish here. I'm not sure about the difference b/w the progesterone creams. I took the kind you by at the health food store a few years back and it was nice. It really seemed to help my cycle and tone down some of my pms symptoms. I wasn't TTC at that time though.
Good luck.