You can certainly get a post-master's certificate in any NP specialty, you just have to take the courses your original MSN was lacking. Question, though: why spend all that time & money to get an MSN in a different specialty to add the FNP later? Post-graduate certificates are not eligible for federal financial aid, so you would have to pay out-of-pocket to get one.
You can also look at this the other way around: it's easy to get the FNP and then get a post-grad subspecialty in education, leadership, informatics, etc.
I would really think about what your main focus is. What do you REALLY want to do with your degree? If your aim is to teach, then stick with MSN in Education. Do you want to work in administration as a Nurse Leader? Then go the MSN Leadership route. Don't get an FNP just to have it. You really need to want to assess, diagnose, treat and follow-up with your patients. If that doesn't appeal to you, I wouldn't waste the time, money and energy on it.
The school I'm attending has post-master's specialty certificates in just about every track of NP specialties, as well as sub-specialty certifications (e.g. cardiology, hospice, education, etc.)