Manage Family Sharing - I have 2 Apple IDs

I want to set up Family Sharing, but I'm not sure how to do it. It seems pretty straight-forward to go into my iCloud settings, but when it tells me to check to be sure the AppleID listed is correct, I see no option to choose a different ID. The problem is that I have an old Apple ID associated with a gmail address that I set up years ago that I've used for my iTunes purchases. I also have an Apple ID with a me.com address that I use for pretty much everything else. I know I can't merge IDs but what are my options?

Is it even possible to switch IDs for Family Sharing? I'm thinking the best way to go may be to set up sharing with my me.com ID, and then add the gmail.com ID as a family member, so that I have 2 personal IDs sharing before I start adding family members. Is there another approach that would be better?