No, they do not have to be minor children if you are all in the same cabin. Anyone of any age can check in with you that is listed in your cabin.
Yes, you can still have separate sail and sign accounts.
The "must be on the same Sail and Sign account" is ONLY when you are talking about separate cabins.....
Thanks LHP for saying that better. I think I may have added to the confusion by just answering the question in the original post.