if you truly want to specialize in senior fitness, I recommend that you check out the Senior Fitness Association http://www.seniorfitness.net/.
While it is true that all organizations have senior fitness conponents, the SFA is entirely focused on that age group.
I personally believe, that if you understand the science of the physiological adaptations that come with aging then there is no need to get another certification.
Certifications like ACE AHFS and the ACSM HFS are really good credentials that put emphasis on the science of aging.
As fitness professionals, the emphasis should always be on how the body responds to a bout of exercise and we need to understand that throughout the entire aging continuum.
Hope this is of help to you.