First of all, this is a wonderful thing you are doing. So many kids are in the foster system and they really need structure and a loving home. Being committed to family reunification is very compassionate of you.
I think you should avoid anything like Mommy or Mama, or anything that sounds like that - it can confuse kids and it can make the legal mom a little defensive when/if reunification is imminent.
I don't think "Nana" is an age thing, but it definitely means a grandmother. The children may, in fact, be reunited with their other nana so that can be problematic in that way as well.
If you think there is a possibility that you may possibly adopt, you will want a name that the kids can continue to use. I think having older kids use whatever they are comfortable with is one thing, but infants and toddlers need to be told what your name is. Still, older kids need to be given a suggestion - perhaps 2 choices - so they aren't sitting there in disarray and confusion and saying, "Great, now I need to figure out how I feel about this person running a household I know nothing about." So there are suggestions of "Ms. Nancy" or something similar.
If you Google "What do foster kids call the foster mom" or something similar, you will get a bunch of websites & blogs - there doesn't seem to be much agreement. The most neutral and consistent seems to be "Auntie" because it's culturally acceptable to so many people, since aunts and uncles are often not biologically connected to the child. It's not threatening to the birth parents but it does imply a strong familial tie. Many of the comments point out that kids just lapse in "Mommy" anyway because the foster mom does "mom things". It's probably wise not to get worked up about what a child calls you on their own, but you definitely need something to suggest, and something that can be signed in some cases.
Also, for infants, you want something easy to pronounce with the sounds they learn first.
Good luck and, again, kudos for all you do.